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ENZYME INHIBITION
AND BIOAPPLICATIONS
Edited by Rakesh Sharma
Enzyme Inhibition and Bioapplications
Edited by Rakesh Sharma
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Enzyme Inhibition and Bioapplications, Edited by Rakesh Sharma
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Contents
Preface IX
Section 1
Basic Concepts
Chapter 1
Enzyme Inhibition: Mechanisms and Scope
Rakesh Sharma
Section 2
Applications of Enzyme Inhibition
Chapter 2
Cytochrome P450 Enzyme Inhibitors from Nature
Simone Badal, Mario Shields and Rupika Delgoda
Chapter 3
Pharmacomodulation of
Broad Spectrum Matrix Metalloproteinase
Inhibitors Towards Regulation of Gelatinases 57
Erika Bourguet, William Hornebeck,
Janos Sapi, Alain Jean-Paul Alix and Gautier Moroy
Chapter 4
Non-Enzymatic
Glycation of Aminotransferases
and the Possibilities of Its Modulation 85
Iva Boušová, Lenka Srbová and Jaroslav Dršata
Chapter 5
Inhibition of Nitric Oxide
Synthase Gene Expression: In vivo Imaging
Approaches of Nitric Oxide with Multimodal Imaging 115
Rakesh Sharma
Chapter 6
Transcriptional Bursting in the Tryptophan Operon of
E. coli and Its Effect on the System Stochastic Dynamics
Emanuel Salazar-Cavazos and Moisés Santillán
Chapter 7
1
3
37
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by
Nitroimidazole and Human Liver Regeneration
Rakesh Sharma
39
195
179
VI
Contents
Chapter 8
Reversible Inhibition of
Tyrosine Protein Phosphatases by Redox Reactions 253
Daniela Cosentino-Gomes and José Roberto Meyer-Fernandes
Chapter 9
Feasible Novozym 435-Catalyzed
Process to Fatty Acid Methyl Ester Production
from Waste Frying Oil: Role of Lipase Inhibition
Laura Azócar, Gustavo Ciudad, Robinson Muñoz,
David Jeison, Claudio Toro and Rodrigo Navia
Chapter 10
Urease Inhibition 303
Muhammad Raza Shah and Zahid Hussain Soomro
277
Preface
Enzyme is a protein molecule exhibiting specific activity and binding affinity with its
substrate molecule to complete enzyme reaction or biocatalytic reaction. Substrate
analogues can inhibit the enzyme reaction and act as enzyme inhibitor. Enzyme
inhibition (Enz-ai-m ie-ni-hi-bi-son) means reducing or blocking an enzyme action on
specific location of enzyme active site by specific substrate or analogue so called
enzyme inhibitor. In modern times, most of the pharmaceutical as well as nutriceutical
compounds are marketed as enzyme inhibitors and such inhibitors exhibit their
specific action in enzyme inhibition inside cells, bacteria, virus, animal plants and
human body. The action of enzyme inhibitors in drug discovery has become a
fundamental approach to pharmacology at any pharmaceutical industry, university
research lab or drug research center. The present issue has been compiled from
various data sources with aim of incorporating a wide range of basic concepts and
applied enzyme inhibition evaluation methods in drug discovery. It is aimed at those
who are embarking on drug discovery research projects, immobilized enzyme solid
state devices as well as relatively experienced pharmacologists, biochemists and
pharmacy scientists who might wish to develop their experiments further to the
advanced level. While it is not possible to detail and include every possible technique
related with enzyme inhibition evaluation in drug design by using specific inhibitors
at specific metabolic mechanism(s), the present issue attempts to provide working tips
with examples and analysis relevant to a wide range of more commonly available
enzyme inhibition techniques.
The methods and concepts described in this book are aimed at giving the reader a
glimpse of some existing enzyme inhibition studies and also methods with context of
each enzyme inhibition method applied for, as well as providing some basis of
familiarizing oneself with these biochemical methods. While enzyme inhibition has
been used as major approach in drug design in the research and industry over last two
decades, it was only later part of 20th century that it has become a major part of so
many applications in biochemical engineering, biomedical engineering of miniatured
clinical chemistry devices in microbiological, bacteriological, immunological,
hormonal testing, nanotechnology, physiological monitoring in health science, plant
science and environment research work. This is, at least in part, due to the continued
development of new solid state polymer platform, pure enzymes and specific
X
Preface
inhibitors available, better understanding of enzyme inhibition mechanisms and
precise detection methods, new awareness of drug discovery and design, with
scanning and monitoring accessories. Thanks to the continued joint efforts of
governmental, industrial and academic institutions globally to promote the need of
new generations of drugs or enzyme inhibitors and new mechanisms of enzyme
inhibition. Regardless of the enzymes or drugs and their brands that are used, one
should always be able to understand and justify the use of right inhibitor or drug
action on enzyme of right choice to drug design for specific study. With this aim,
different approaches of enzyme inhibition methods are presented in separate chapters
on the use of different enzyme inhibitors. For learners, basic concepts, mechanistic
issues, limitations in drug testing, skepticism in enzyme inhibition approach and drug
variability due to non-specific analogues of enzyme inhibitors or substrates is
presented with a working enzyme inhibition protocols for drug design and analysis of
their inhibitory action.
In chapter 1, the authors have introduced the basic concepts on enzyme, enzyme
reaction, inhibitors and types of inhibition with a handful established applications in
drug discovery, immobilized enzyme engineering, and biosensing. Major three basic
types of enzyme inhibition kinetics is highlighted for beginners. Examples of substrate
analogues and their enzyme inhibition behaviour are illustrated with color schemes.
Application of immobilized enzyme on chips for environment monitoring and
biosensor development is quite intriguing for engineers, scientists and industrialists.
In chapter 2, authors overviewed isolates from Pepermia amplexicaulis and Spathelia
sorbifolia plants have examined for CYP inhibitions to exhibit antiprotozoal,
chemopreventive and anti-cancer activity. Other plant sources of tea and fruits of
Rhytidophyllum tomentosa, Psidium guajava, Symphytium officinale, Momordica charantia
showed inhibition properties of these teas against a panel of CYP450 enzymes in order
to assess the potential for drug interactions with co-medicated pharmaceuticals. The
chapter highlights the potential of a few natural products emanating from the
Caribbean: chromene amides isolated from Amyris plumieri, quassinoids isolated from
Picrasma excelsa, anhydrosorbifolin isolated from Spathelia sorbifolia and 5-Hydroxy-2,7dimethyl-8-(3-methyl-but-2-enyl)-2-(4-methyl-penta-1,3-dienyl)-chroman-6-carboxylic
acid isolated from Peperomia amplexicaulis. New information is presented on bioactive
screens against CYP enzymes in the presence of five aqueous infusions of popularly
used herbs; Rhytidophyllum tomentosa, Psidium guajava, Symphytium officinale, Momordica
charantia and Picrasma excelsa. Authors emphasized search of CYPs 1A1 and 1B1 activity
inhibitors as chemoprotectors such as CA1 and quassin for CYP 1A1; and
anhydrosorbifolin and 5-Hydroxy-2,7-dimethyl-8-(3-methyl-but-2-enyl)-2-(4-methylpenta-1,3-dienyl)-chroman-6-carboxylic acid for CYP 1B1 in search of safer herbal
remedies in co-adiministration of medicines in cancer prevention. In chapter 3, authors
described pharmacomodulation in metalloprotease enzymes by inhibitors. Authors
aim to achieve better metallloprotease (MMP) enzyme inhibitors with good MMP-2
selectivities to increase hydrophobicity and rigidity with the dehydro and didehydro
analogues and synthesized (analogue 2a-d and 3a-h). Authors displayed sevral
Preface
examples of inhibitors including pharmacomodulation of galardin®, a powerful broad
spectrum MMPI, hydrazide and sulfonylhydrazide-type functions as potential Zinc
Binding Compounds for gelatinase enzyme inhibition, double-headed elastase-MMP
inhibitor (d-hEMI) able to block elastase and MMP activities, with display of
structural-functional models in the last sections. Authors speculate to develop hybrid
nanoprobes build from MMPI and fluorescent nanocrystal quantum dots (QDs).
Design and chemical synthesis of derivatives of galardin®, selective inhibitors of MMP2, tagging with QDs. In hope of photo- and chemical stability of QDs, it is possible that
apporoach will enable long-term spatiotemporal tracking of the process of inhibition
of MMP-2 enzymes to offer better understanding of physiological process of invasion
of melanoma. In chapter 4, authors introduce the readers with nonenzymatic glycation
of aminotransferases and modulation of these enzymes as model protein. Further
authors emphasized the significance of advanced glycation end-products with
introduction of structure, targeting with aim to evaluate potential antiglycation
activity of two mitochondrial antioxidants, α-phenyl N-tert-butyl nitrone (PBN) and Ntert-butyl hydroxylamine (NtBHA). In next section, authors established the effect of
natural compound on fructose induced AST glycation, basic techniques to determine
primary amino groups, enzyme molecular charge of AST modulated by fructose by
electrophoresis. Authors attempted to search the compounds with antioxidant and
potential antiglycating activities in an illustrated description with a perspective of
their use as remedies against diabetic complications. In chapter 5, author introduces
the readers with basic mechanism of reduced nitric oxide synthase gene expression
and applications of nitric oxide synthase (NOS) inhibition and approaches of nitric
oxide (NO) content by using less known multimodal imaging. Nitric oxide imaging
techniques utilize mapping NO in tissue using NO specific imaging contrast agents
sensitive to fluorescence, magnetic resonance and electron spin resonance. A handful
account is presented on NOS expression inhibitors, possibility of dithiacarbamates,
paramagnetic complexes for bioimaging of NO. For learners, MRI protocol is given as
example. In the light of recent developments in multimodal bioimaging of NO/NOS
expression by bioluminescence, fluorescence techniques, a handful information is
given in cells, animals, plants and humans body in different diseases including
endothelial cell injury, apoptosis, renal, liver, lung, muscle, brain, inflammation,
bones, retina with emphasis on multimodal techniques of calcium, ion channels, iron
bound complexes. Author speculated the future possibility of NO/NOS bioimaging by
combined radioimaging techniques and it remains to see if real-time imaging becomes
routine modality. In chapter 6, transcriptional bursting in E.coli tryptophan operon is
introduced to explain stochastic dynamics of the tryptophan synthase enzyme
feedback inhibition regulatory mechanism at various levels of Trp operon genes.
Authors have described in chapter various components of trp operon structure, model
development, parameter estimation, and numerical methods with results on stochastic
stimulations to evidence the least response time with inhibition-less strain. Authors
further investigated that the repressor-operator interaction stimulates transcriptional
bursting which shows several dynamic effects on transcriptional bursting possibly by
a feedback enzyme-inhibition regulatory mechanism. In chapter 7, a hepatocellular
XI
XII
Preface
dysfunction criteria with possible mechanism of hepatocellular dysfunction is
proposed to evaluate liver degeneration due to amoebic infection and hepatic recovery
by nitroimidazole administration. The main focus is the evaluation of regulatory
enzyme inhibition effects on major energy metabolism. Liver regeneration is an
excitement to reverse the process of enzyme inhibition in defense. Author introduces
enzymes in liver abscess, programmed cell death, hepatocellular criteria of hypoxia,
loss of metabolic integrity with low NADH/ATP, parenchymal lysosomal enzyme
inhibition illustrated by regulatory behavior changes of glucokinase,
phiosphofructokinase, pyruvate kinase, phosphodiesterase, respiratory burst,
superoxide dismutase, cytochrome oxidase, adenylate cyclase, inhibition of drug
metabolizing microsomal enzymes, phagocytosis, and DNA synthesis. A model is
proposed on proteolysis in isolated lysosomes to establish the mechanism of
degradation and proteolysis inhibition. Further, role of lysosomal enzymes is
proposed for liver regeneration and recovery after nitroimidazole treatment. The
emerging state of art is presented on role of enzyme inhibition in liver transplantation
and tissue engineering. Still, art of liver regeneration is not free from several
challenges such as lack of ideal stimulator of hepatic recovery. In attempt to stimulate
the liver regeneration by nitroimidazole, it was explored that nitroimidazole is
becoming a multi-organ therapeutic drug for tumor, tuberculosis, myocardial
infarction, hypoxia and diagnostic tool in imaging, chemosensor, and tissue
engineering in addition to hepatic recovery. In chapter 8, authors describe reversal
inhibition of tyrosine protein phosphatases and explore it by redox reactions.
Investigators describe a structural mechanistic action of ROS in the tyrosine
phosphatase enzymatic activity to demonstrate how it interacts with their target
molecules; the reversible regulation of this enzyme by oxidants and antioxidants; and
the major consequences of this tightly controlled mechanism on cell signalling.
Authors describe tyrosine phosphatase family, catalytic domain of tyrosine
phosphatase, sources of redox radical, reactive nitrogen, antioxidant mechanisms,
redox inhibition, and mechanism of cellular signalling. Authors further emphasize an
urgent need of a mechanism(s) available on specific tyrosine phosphatase action to
identify the sources of reactive radical species formation. In chapter 9, a novel
technique ‘response surface methodology’ is proposed using lipase inhibition by
Novozyme 435-catalyse process to produce fatty acid methyl ester from waste frying
oil. The issue of preserving lipase in the reaction is addressed without any loss of
enzyme activity due to methanol in the reactor. The chapter describes high yield lipase
catalysed process, optimization conditions, properties of feedstock, economic biodiesel
production with lipase reutilization, and kinetics of tert-butanol system. Authors
highlighted several issues such as short reaction time, flexible enzyme process, and
utilization of low cost raw materials as environment-friendly chemical catalysis in
economic enzyme process. In chapter 10, author introduces urease inhibition, chemical
reactions of urease organic, metal inhibitors, and urease as virulence factor for the
urinary tract infections, gastrointestinal infection by inhibition of urease. Author
describes urease inhibition mechanism, kinetics, action of inhibition, potential
inhibitors, and structure activity relations. Author speculates the potentials of
Preface
inhibitors in biomedical science with more applications in pharmaceutical, agriculture,
environmental research such as sensors, adsorbent and commercial devices.
In last three decades, science of enzyme inhibition has grown by leaps and bounds in
the field of biochemistry, medicine and pharmacology. With tremendous
developments in technology, now enzymes are valuable research tools as biomarkers,
biosensors, detection miniature biodevices, point-of-care pocket monitors in
emergency care. Use of enzyme inhibitors in valuable drug testing has rocked the
business in pharmaceutical industries and biotechnology industries in search of
suitable and effective drug. Present time, science has grown mainly in three directions.
First, precision and accuracy in measurement of enzyme reaction has made possible to
measure and localize the site of enzyme inhibition at the level of picoscale to down
side of 10-12 meters. For example, enzyme reaction of caspases and metalloproteases
can be detected and visualized by vision® at the site of tumor under powerful
microscope. In future, it is speculated that picotechnology will replace
nanotechnology in terms of more detectable applications at picoscale instead of
nanoscale with better understanding of enzyme-substrate or inhibitor complexes.
Further, solid phase enzyme devices or immobilized enzyme fixed on polymeric base
have emerged as detection tools in agriculture, environment, healthcare, and
biotechnology industries.
Second, enzyme inhibitors do behave very precise at the exact location of enzyme
active site in the ‘lock-key’ configuration. The 3D structure and conformation of active
site permits very powerful enzyme inhibitors or substrate analogues to bind with
enzyme molecule to make desirable product and accomplish the goal as
pharmaceutical drug. Many drug classes and variants have been explored and
developed for almost each and every disease, infection and immunity. Better sources
of structural-functional relationship enzyme-inhibitor and database available have
changed the scenario. Now it is a fashion of new generation medication available
based on new drug discovery almost every year. It is all possible due to new
developments in drug testing or enzyme inhibitor development in pre-clinical and
clinical use.
Third, enzyme inhibitors play a major role in enzyme engineering and many other
related fields. Design of portable devices using enzyme reaction as detection
mechanism need a polymer base with layer of enzyme molecules fixed on its surface
and placed in device at the point-of-detection. Such miniature detection devices are
becoming popular in detection of microbial, bacterial contamination, pollutes, organic,
chemical toxicity, allergens, harmful gases, paints, vapors, bioconversion recovery,
molecular biology products, biopharmaceutical products, hormones, agroproducts,
nutraceutical products, and list is growing in use at clinical chemistry, microbiology,
toxicology, pathology labs and quick diagnostic tools in hospitals.
XIII
XIV Preface
I acknowledge Anja Filipovic for the guidance in book production, the continuous
efforts made by Miss Nisha Keval to assist me in whole editorial work, in shaping the
chapters at various points of time in presentable form, and Professor P.V.Pannirselvam
from Brazil, Federal University Riogrande Norte, Natal, Brazil in chapter 1.
Finally, I thank all authors and co-authors for their chapter contributions and for
selecting the right topics suitable for this book.
I hope that readers will enjoy reading the book and that it will serve the purpose of
basic concepts on enzyme inhibition with advanced applications in the field of applied
enzyme inhibition in pharmaceutical, biotech industries and academic research.
Rakesh Sharma, Ph.D
MS-Ph.D, ABR II
Professor (Nanotechnology)
Amity University,
India
Research Professor, Center of Nano-Biotechnology,
Florida State University, Tallahassee, FL,
USA
Section 1
Basic Concepts
1
Enzyme Inhibition: Mechanisms and Scope
1Center
Rakesh Sharma1,2,3
of Nanomagnetics Biotechnology, Florida State University, Tallahassee, FL
2Innovations and Solutions Inc. USA, Tallahassee, FL
3Amity University, NOIDA, UP
1,2USA
3India
1. Introduction
Enzyme is a protein molecule acting as catalyst in enzyme reaction. Enzyme inhibition is a
science of enzyme-substrate reaction influenced by the presence of any organic chemical or
inorganic metal or biosynthetic compound due to their covalent or non-covalent interactions
with enzyme active site. It is well known that all these inhibitors follow same rule to
interplay in enzyme reaction. Present chapter introduces beginners with basic tenets of
classic presumptions of enzyme inhibition, types of enzyme inhibitors, different models of
enzyme inhibition with established examples cited in literature, and scientific basis of
emerging immobilized enzyme technology in different applications. In the end, limitations
of using classic presumptions and variants of enzyme inhibition are highlighted with new
challenges to achieve best results. Present time, best approach is 'customize new technology
with detailed analysis to make it highly efficient' in both drug discovery and enzyme
biosensor industry. However, other applications are described in following chapters on
pesticides, herbicides.
2. What are enzyme inhibitors?
The enzyme inhibitors are low molecular weight chemical compounds. They can reduce or
completely inhibit the enzyme catalytic activity either reversibly or permanently
(irreversibly). Inhibitor can modify one amino acid, or several side chain(s) required in
enzyme catalytic activity. To protect enzyme catalytic site from any change, ligand binds
with critical side chain in enzyme. Safely, chemical modification can be done to test inhibitor
for any drug value.
In drug discovery, several drug analogues are chosen and/or designed to inhibit specific
enzymes. However, detoxification or reduced toxic effect of many antitoxins is also
accomplished mainly due to their enzyme inhibitory action. Therefore, studying the
aforementioned enzyme kinetics and structure-function relationship is vital to understand
the kinetics of enzyme inhibition that in turn is fundamental to the modern design of
pharmaceuticals in industries [Sami et al. 2011]. Enzyme inhibition kinetics behavior and
inhibitor structure-function relationship with enzyme active site clarify the mechanisms of
4
Enzyme Inhibition and Bioapplications
enzyme inhibition action and physiological regulation of metabolic enzymes as evidenced in
following chapters in this book. Some notable classic examples are: drug and toxin action
and/or drug design for therapeutic uses e.g., iodoacetamide deactivates cys amino acid in
enzyme side chain; methotrexate in cancer chemotherapy through semi-selectively inhibit
DNA synthesis of malignant cells; aspirin inhibits the synthesis of the proinflammatory
prostaglandins; sulfa drugs inhibit the folic acid synthesis essential for growth of pathogenic
bacteria and so many other drugs. Many life-threatening poisons, e.g., cyanide, carbon
monoxide and polychlorinated biphenols are all enzyme inhibitors.
Conceptually, enzyme inhibitors are classified into two types: non-specific inhibitors and
specific inhibitors.
The enzyme inhibition reactions follow a set of rules as mentioned in following rules.
Presently, computer based enzyme kinetics data analysis softwares are developed using
following basic presumptions.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Enzyme interacts with substrate in 1:1 ratio at active site to catalyze the reaction.
Enzyme binds with substrate at active site in the form of a lock-key 3D arrangement for
induced fit.
Inhibitor active groups compete with substrate active groups and/or active groups at
enzyme allosteric catalytic site in a synergistic manner or first cum first preference
(competition) to make enzyme-inhibitor-substrate/enzyme-substrate/enzyme-inhibitor
complexes.
Enzyme-inhibitor-substrate complex formation depends on active free energy loss and
thermodynamic principles.
Enzyme and substrate or inhibitors react with each other as active masses and reaction
progresses in kinetic manner of forward or backward reaction.
Kinetic nature of inhibitor or substrate binding with enzyme is expressed as kinetic
constants of a catalytic reaction.
Enzyme reaction(s) are highly depend on physiological conditions such as pH,
temperature, concentration of reactants, reaction period to determine the rate of reaction.
Substrate and inhibitor molecules arrange over enzyme active site on specific sub
unit(s) in 3D manner. As a result enzyme-substrate-inhibitor exhibit binding rates
depend on allosteric sites or subunit-subunit homotropic or heterotropic interactions.
Intermolecular forces between enzyme subunits, substrate or inhibitor active group
interactions, physical properties of binding nature: electrophilic, hydrophilic, nucleophilic
and metalloprotein nature; hydrogen bonding affect the overall enzyme reaction rates and
mode of inhibition (3D orientation of inhibitor molecule on enzyme active site).
Other factors are also significant in determining enzyme inhibition reaction as described in
each individual inhibitor in following sections. For basic principles of enzyme units
(apoenzyme, holoenzyme, co-factor, co-enzyme) in enzyme catalysis, active energy loss,
Michaelis-Menton Equations, LeChatelier’s principle, Lineweaber-Burk and semi-log plots,
apparent and actual plots, readers are requested to read text books [Schnell et al. 2003,
Nelson, et al. 2008, Jakobowski 2010a, Strayer et al. 2011]. Our focus is enzyme inhibition
mechanisms with examples in following description. For multisubstrate enzymes, pingpong mechanism, allosteric mechanisms, and diffusion kinetics, readers are requested to
read original papers [Pryciak 2008, Bashor 2008, Jakobowski 2010b]
Enzyme Inhibition: Mechanisms and Scope
5
These inhibitors may act in reversible or irreversible manner. Non-specific irreversible noncompetitive inhibitors include all protein denaturating factors (physical and chemical
denaturation factors). The specific inhibitors attack a specific component of the holoenzyme
system. The action depends on increased amount of substrate or by other means of
physiological conditions, toxins. Specific inhibitors can be described in several forms
including; 1) coenzyme inhibitors: e.g., cyanide, hydrazine and hydroxylamine that inhibit
pyridoxal phosphate, and, dicumarol that is a competitive antagonist for vitamin K; 2)
inhibitors of specific ion cofactor: e.g., fluoride that chelates Mg2+ of enolase enzyme; 3)
prosthetic group inhibitors: e.g., cyanide that inhibits the heme prosthetic group of cytochrome
oxidase; and, 4) apoenzyme inhibitors that attack the apoenzyme component of the
holoenzyme; 5) physiological modulators of reaction pH and temperature that denature the
enzyme catalytic site.
The apoenzyme inhibitors are of two types; i) Reversible inhibitors; their inhibitory action is
reversible because they make reversible association with the enzyme, and, ii) Irreversible
inhibitors; because they make inactivating irreversible covalent modification of an essential
residue of the enzyme. Apoenzyme inhibitors show effect on Km and Vmax. The reversible
apoenzyme inhibitors are also called metabolic antagonists. They are of three subtypes; a)
competitive, b) uncompetitive and c) non-competitive or mixed type. For example: enzyme
inhibitors are used in drug design.
Discovery of useful new enzyme inhibitors used to be done by trial and error through
screening a huge library of compounds against a target enzyme at allosteric catalytic site.
This approach is still in use for compounds with combinatorial chemistry and highthroughput screening technology as described in following description based on recent
concepts [El-Metwally et al. 2010]. However, rational drug design as an alternative approach
uses the three-dimensional structure of an enzyme's active site or transition-state
conformation to predict which molecules might be ideal inhibitors as given an example of
urease in chapter 11 in this book. 3D-structure shortens the long screening list towards a
right set of novel inhibitor which kinetically characterizes and allows specific structural
changes in amino acids of catalytic site chain to optimize inhibitor-enzyme binding.
Alternatively, molecular docking and molecular mechanics are computer-based methods
that predict the affinity of an inhibitor for an enzyme. In following description, a glimpse of
these mechanisms is given on different types of inhibitors based on recent classic book [ElMetwally et al. 2010]. Readers are requested to read other classic details from advanced text
books [Dixon and Webb, 1979].
3. Irreversible inhibition
The irreversible apoenzyme inhibitors have no structural relationship to the substrate and
bind covalently. They also bind stable non-covalently with the active site of the enzyme or
destroy an essential functional group of active site. So, irreversible inhibitors are used to
identify functional groups of the enzyme active sites at which location they bind. Although
inhibitors have limited therapeutic applications because they are usually act as poisons. A
subset of irreversible inhibitors called suicide irreversible inhibitors, are relatively inactive
compounds. They get activated upon binding with the active site of a specific enzyme. After
such binding, the suicide irreversible inhibitor is activated by the first few intermediary
6
Enzyme Inhibition and Bioapplications
steps of the biochemical reaction - like the normal substrate. However, it does not release
any product because of its irreversible binding at the enzyme active site. Inhibitors make use
of the normal enzyme reaction mechanism to get activated and subsequently inactivate the
enzyme. Due to this very nature, suicide irreversible inhibitors are also called mechanismbased inactivators or transition state analog inhibitors. Thus, inhibitor exploits the transition
state stabilizing effect of the enzyme, resulting in a better binding affinity (lower Ki) than
substrate-based designs. An example of such a transition state inhibitor is active form of the
antiviral drug oseltamivir (Tamiflu; see Figure 1); this drug mimics the planar nature of the
ring oxonium ion in the reaction of the viral enzyme neuraminidase [El-Metwally et al.
2010]. After drug activation in the liver, the drug replaces sialic acid as the normal substrate
found on the surface proteins of normal host cells. It prevents the release of new viral
particles from infected cells. It has been used to treat and prevent Influenza virus A and
Influenza virus B infections. Most of such inhibitors are classified as tight-binding
competitive inhibitors in other references of enzymes. However, their reaction kinetics is
essentially irreversible.
O
O
HN
O
H 2N
Fig. 1. The transition state analog oseltamivir - the viral neuraminidase inhibitor.
The present art of drug discovery and design of new drugs is based on suicidal irreversible
inhibitors. Chemicals are synthesized based on knowledge of 3D conformation of substrateactive site binding at specific binding rates in presence of co-factors, co-enzyme (enzyme
reaction mechanisms) to inhibit at specific enzyme active site with minimal side-effects due
to its non-specific binding nature. Transition state analogs are extremely potent and specific
inhibitors of enzymes because they have higher affinity and stronger binding to the active
site of the target enzyme than the natural substrates or products. However, exact design of
drugs that precisely mimic the transition state is a challenge because of unstable structure of
transition state in the free-state. Prodrugs undergo initial reaction(s) to form an overall
electrostatic and three-dimensional intermediate transition state complex form with close
similarity to that of the substrate. These prodrugs serve as guideline for drug development
to form transition state suitable for stable modification; or, using the transition state analog
to design a complementary catalytic antibody; called Abzyme. Example: Abzymes are used
in catalytic antibodies and ribozymes in catalytic ribosomes [El-Metwally et al. 2010].

Abzymes are antibodies generated against analogs of the transition state complex of a
specific chemical. The arrangement of amino acid side chains at the abzyme variable
regions is similar to the active site of the enzyme in the transition state and work as
artificial enzymes. For example, an abzyme was developed against analogs of the
transition state complex of cocaine esterase, the enzyme that degrades cocaine in the
body [El-Metwally et al. 2010]. Thus, this abzyme has similar esterase activity that is
Enzyme Inhibition: Mechanisms and Scope





7
used as injection drug to rapidly destroy cocaine in the blood of addicted individuals to
decreasing their dependence on it.
Thrombin inhibition is common in saliva of leeches and other blood-sucking organisms.
They contain the anticoagulant hirudin that irreversibly inhibits thrombin, and, to
regain thrombin action synthesis of new thrombin molecules is required. This made it
unsafe as an anticoagulation drug. However, based on hirudin structure, rational drug
design synthesized 20-amino acids peptide known as bivalirudin that is safe for longterm use because of its reversible effects on thrombin; despite its high binding affinity
and specificity for thrombin.
Ornithine decarboxylase by difluoromethylornithine is used to treat African
trypanosomiasis (sleeping sickness). The enzyme initially decarboxylates
difluoromethylornithine instead of ornithine and releases a fluorine atom, leaving the
rest of the molecule as a highly electrophilic conjugated imine. The later reacts with
either a cysteine or lysine residue in the active site to irreversibly inactivate the enzyme.
Inhibition of thymidylate synthase by fluoro-dUMP. Imidazole antimycotic drugs are
examples of such group that inhibit several subtypes of cytochrome P450 [Sharma,
1990]. The mechanisms of toxicities and antidotes of irreversible inhibitors are of
medical pathological importance. Because of the irreversible inactivation of the enzyme,
irreversible inhibition is of long duration in the biological system because reversal of
their action requires synthesis of new enzyme molecules at the enzyme genetranscription-translation level.
Inhibition of acetylcholine esterase (ACE) by diisopropylfluorophosphate (DPFP), the
ancestor of current organophosphorus nerve gases (e.g., Sarin and Tabun) and other
organophosphorus toxins (e.g., the insecticides Malathion and Parathion and
chlorpyrifos). ACE hydrolyzes the acetylcholine into acetate and choline to terminate
the transmission of the neural signal form the neuromuscular excitatory acetylcholine
presynaptic cell to somatic neuromuscular junction (see Figure 2). DPFP as a potent
neurotoxin inhibits ACE and acetylcholine hydrolysis. Failure of hydrolysis leads to
persistent acetylcholine excitatory state and improper vital function particularly
respiratory muscles that may lead to suffocation; with a lethal dose of less than 100 mg.
DPFP inhibits other enzymes with the reactive serine residue at the active site, e.g.,
serine proteases such as trypsin and chymotrypsin, but the inhibition is not as lethal as
that of acetylcholine esterase. Similar to DPFP, malaoxon the toxic reactive derivative
from Malathion (after its metabolism by the liver) binds initially reversibly and then
irreversibly (after dealkylation of the inhibitor) to the active site serine and inactivates
ACE and other enzymes. Lethal doses of oral Malathion are estimated at 1 g/kg of body
weight for humans.
Inhibition of ACE by these poisons leads to accumulation of acetylcholine that overstimulates the autonomic nervous system (including heart, blood vessels, and glands),
thereby accounting for the poisoning symptoms of vomiting, abdominal cramps,
nausea, salivation, and sweating. Acetylcholine is also a neurotransmitter for the
somatic motor nervous system, where its accumulation resulted in poisoning symptom
of involuntary muscle twitching (muscle fasciculation), convulsions, respiratory failure
and coma. Intoxication of Malathion is treated by the antidote drug Oxime that
reactivates the acetylcholine esterase and by intravenous injection of the anticholinergic
(antimuscarinic) drug atropine to antagonize the action of the excessive amounts of
acetylcholine [El-Metwally et al. 2010].
8
Enzyme Inhibition and Bioapplications
O
H3C
CH
P
CH3
O CH
CH3
H3C
F
Diisopropylfluorophosphate
H3C
O
H3C
O
S
P
C
O
CH2
H2C
C
O
CH2 CH3
O
CH3
S
O
H3C
CH2
O
H3C
C O CH2 CH2 N+ CH3
Acetylcholine
O Acetylcholine CH3
esterase
S
P
H2O H3C COOH Acetate
H3C
CH
CH3
CH3
CN
CH3
CH
CH3
CH3
H3C
HF
O
CH
N
HO CH2 CH2 N+ CH3
Choline
CH3
ACE
F P O DPFP
H3C
O
P
F
Sarin
Parathion
CH3
O
Tabun
O
NO2
Serine CH2 OH
CH2
CH3
CH2
Active
acetylcholine ACE
esterase
H3C
O
Malathion
H3C
P
CH3
O
S CH
CH3
CH
CH3
O
Inhibited
Serine CH2 P O acetylcholine
H3C
CH
esterase
CH3
Fig. 2. Organophosphorus compounds and the suicidal irreversible mechanism-based
inhibition of the enzyme acetylcholine esterase by diisopropylfluorophosphate. Malathion
and parathion are organophosphorus insecticides. The nerve gases Tabun and Sarin are
other organophosphorus compounds.
Another example of irreversible inhibition is iodoacetate inhibition of the glycolytic
glyceraldehyde-3-phosphate dehydrogenase (GPD). Iodoacetate is a sulfhydryl compound
that covalently alkylates and blocks the sulfhydryl group at the active site of the enzyme.
Iodoacetate also inhibits other enzymes with -SH at the active site (Figure 3).
IH
GPD Cysteine CH2 SH I CH2 COOH
Active glyceraldehyde-3-phosphate Iodoacetate
dehydrogenase
GPD Cysteine CH2 S CH2 COOH
Inhibited glyceraldehyde-3-phosphate
dehydrogenase
Fig. 3. The suicidal irreversible mechanism-based inhibition of the enzyme glyceraldehyde3-phosphate dehydrogenase by iodoacetate.

Allopurinol - the anti-gout drug - is a suicidal irreversible mechanism-based inhibitor of
the enzyme xanthine oxidase that works as oxidase or dehydrogenase. The enzyme
commits suicide by initial activating allopurinol into a transition state analog oxypurinol - that bind very tightly to molybdenum-sulfide (Mo-S) complex at the active
site (Figure 4). This enzyme accounts for the human dietary requirement for the trace
mineral molybdenum. The molybdenum-sulfide (Mo-S) complex binds the substrates
and transfers the electrons required for the oxidation reactions.
9
Enzyme Inhibition: Mechanisms and Scope
O
O
N
HN
N
Xanthine oxidase
H 2O + H +
N
H
O
N
(Mo=S) HN
3H+ + 2e-
O
N
H
N
H
Hypoxanthine
Xanthine oxidase (Mo=S)
H 2O + H +
3H+ + 2e-
HN
O
HN
O
H
C
N
N
N
H
H
C
Xanthine oxidase (Mo=S) HN
H 2O + H +
3H+ + 2e-
O
N
N
H
N
H
Uric acid
Xanthine oxidase (Mo=S);
inactive complex
N
H
N
H
Allopurinol
O
O
Xanthine
to O2 to give H2O2 (Oxidase), or,
to O2 to give H2O2 (Oxidase), or,
to NAD+ to give NADH.H+ (Dehydrogenase)
to NAD+ to give NADH.H+ (Dehydrogenase)
H
N
Oxypurinol
to O2 to give H2O2 (Oxidase), or,
to NAD+ to give NADH.H+ (Dehydrogenase)
Fig. 4. The suicidal irreversible mechanism-based inhibition of the enzyme xanthine oxidase
by allopurinol.

Guanosine analogue antiviral drug aciclovir - acycloguanosine (2-amino-9-((2hydroxyethoxy)methyl)-1H-purin-6(9H)-one), as one of the most commonly-used
antiviral drugs, it is primarily used for the treatment of herpes simplex and herpes
zoster (shingles) viral infections. Aciclovir (see Figure 5) started a new era in antiviral
therapy, as it is extremely selective and low in cytotoxicity. Aciclovir as a prodrug
differs from previous nucleoside analogues in that it contains only a partial nucleoside
structure: the sugar ring is replaced by an open-chain structure. It is selectively
converted into acyclo-guanosine monophosphate (acyclo-GMP) by viral thymidine
kinase, which is far more effective (3000 times) in phosphorylation than cellular
thymidine kinase. Subsequently, the monophosphate form is further phosphorylated
into the active triphosphate form, acyclo-guanosine triphosphate (acyclo-GTP), by
cellular kinases. Acyclo-GTP is a very potent inhibitor of viral DNA polymerase; it has
approximately 100 times greater affinity for viral than cellular polymerase. As a
substrate, acyclo-GTP is incorporated into viral DNA, resulting in chain termination.
Acyclo-GTP is fairly rapidly metabolized within the cell, possibly by cellular
phosphatases.
O
N
HN
H2N
N
N
O
OH
Aciclovir
Fig. 5. Aciclovir; the prodrug for the suicidal irreversible inhibition of the viral DNA
polymerase.

The antibiotic penicillin is another transition state analog suicidal inhibitor that binds
irreversibly covalently to serine at the active site of the bacterial enzyme glycopeptide
transpeptidase. The enzyme is a serine protease required for synthesis of the bacterial
cell wall and is essential for bacterial growth and survival. It normally cleaves the
10
Enzyme Inhibition and Bioapplications
peptide bond between two D-alanine residues in a polypeptide. Penicillin structure
contains a strained peptide bond within the β-lactam ring that resembles the transition
state of the normal cleavage reaction, and thus penicillin binds very readily to the
enzyme active site. The partial reaction to cleave the imitating penicillin peptide bond
activates penicillin to bind irreversibly covalently to the active site serine (Figure 6).
R
C O
NH
HC
Strained peptide bond C
HO - Serine-Glycopeptide Transpeptidase;
Free and active
O
R
Penicillin
S
CH
C
N
C
H
C O
CH3
NH
HC
CH3
COO- O C
CH3
S
CH
HN
C
C
H
CH3
COO-
O - Serine-Glycopeptide Transpeptidase;
Covalently bound and inactive
Fig. 6. The suicidal irreversible mechanism-based inhibition of the bacterial enzyme
glycopeptide transpeptidase by the antibiotic penicillin.


Aspirin (acetylsalicylic acid) provides an example of a pharmacologic drug that exerts its
effect through the covalent acetylation of an active site serine in the enzyme
cyclooxygenase (prostaglandin endoperoxide synthase). Aspirin resembles a portion of
the prostaglandin precursor that is a physiologic substrate for the enzyme.
Heavy metal toxicity is caused by tight binding of a metal such as mercury, lead,
aluminum, or iron, to a functional group at the active site of an enzyme. At high
concentration of the toxin, heavy metals are relatively nonspecific for the enzymes they
inhibit and inhibit a large number of enzymes. For example, it is impossible to specify
which particular enzyme is implicated in mercury toxicity that binds reactive -SH
groups at the active sites. Lead developmental and neurologic toxicity is caused by its
ability to replace the normal functional metal in target enzymes; particularly Ca2+ in
important enzymes, e.g., Ca2+-calmodulin and protein kinase C. Because of their
irreversible effect, heavy metals are routinely use as fixatives in histological
preparations.
Kinetically, the irreversible inhibitors decrease the concentration of active enzyme and in
turn decrease the maximum possible concentration of ES complex with ultimate reduction
in the reaction rate of the inactivated individual enzyme molecules. The remaining
unmodified enzyme molecules are normally functional considering their turnover number
and Km. For example: Natural poisons act as Enzyme inhibitors and Inhibitory enzymes
In nature, animals and plants are rich in poisons as secondary metabolites, peptides and
proteins that can act as enzyme inhibitors. Natural toxins are small organic molecules and
act as natural inhibitors for enzymes in metabolic pathways and non-catalytic proteins.

Neurotoxins are natural inhibitors, toxic but valuable for therapeutic uses at lower
doses. For example, glycoalkaloids from Solanaceae family plants (potato, tomato and
eggplant) act as acetylcholinesterase inhibitors to increase the acetylcholine
neurotransmitter, muscular paralysis and then death. Many natural toxins are
secondary metabolites. These neurotoxins also include peptides and proteins. An
example of a toxic peptide is alpha-amanitin, found in death cap mushroom and acts
Enzyme Inhibition: Mechanisms and Scope
11
potent enzyme inhibitor, in this case preventing the RNA polymerase II enzyme from
transcribing DNA. The algal toxin microcystin is also a peptide and is an inhibitor of
protein phosphatases. This toxin can contaminate water supplies after algal blooms and
is a known carcinogen that can also cause acute liver hemorrhage and death at higher
doses. Proteins can also be natural poisons or antinutrients, such as the trypsin
inhibitors that are found in some legumes, potato, and tomato. Several invertebrate and
vertebrate venoms contain protein and peptide enzyme inhibitors for, e.g., plasmin,
renin and angiotensin converting enzymes. Inhibitory enzymes are enzymes that
irreversibly inhibit other enzymes by chemically modifying them. In the broad sense,
they include all proteases and lysosomal enzymes. Some of them are toxic plant
products, e.g., ricin, a glycosidase that is an extremely potent protein toxin found in
castor oil beans. It inactivates ribosomes by cleavage the eukaryotic 28S rRNA and
reduces protein synthesis and a single molecule of ricin is enough to kill a cell.
4. Reversible inhibition
Reversible inhibitors may be competitive, noncompetitive, or uncompetitive inhibitors
relative to a particular substrate. Products of enzymatic reactions are reversible inhibitors of
the enzymes. A decrease in the rate of an enzyme caused by the accumulation of its own
product plays an important role in the balance and most economic usage of metabolic
pathways. It prevents one enzyme in a sequence of reactions from generating a new product
more than the capacity of the next enzyme in that sequence, e.g., inhibition of hexokinase by
accumulating glucose 6-phosphate.
With the reduction in the inhibitor concentration, the enzyme activity is regenerated due to
the non-covalent association and the reversible equilibrium with the enzyme. The
equilibrium constant for the dissociation of enzyme inhibitor complexes is known as Ki that
equals [E][I]/[EI] [Cheng et al. 1973]. The inhibition efffect of Ki on the reaction kinetics is
reflected on the normal Km and or Vmax observed in Lineweaver-Burk plots; in a pattern
dependent on the type of the inhibitor [Nelson et al. 2008]. The inhibitor is removable by
several ways. The three common types of reversible inhibitions are:



Competitive reversible inhibition.
Uncompetitive reversible inhibition.
Mixed reversible inhibition (or non-competitive inhibition).
4.1 Competitive reversible inhibition
The competitive inhibitor is structurally related to the substrate and binds reversibly at the
active site of enzyme and occupies it in a mutually exclusive manner with the substrate.
Therefore, the competitive inhibitor competes with the substrate for the active site. The
binding is mutually exclusive because of their free competition. According to the law of
mass action, relatively higher inhibitor concentration prevents the substrate binding. Since
the reaction rate is directly proportional to [ES], reduction in ES formation for EI formation
lowers the rate. Increasing substrate towards a saturating concentration alleviates
competitive inhibition. In the time enzyme-substrate complex releases the free enzyme and a
product, the enzyme-inhibitor complex does release neither free enzyme nor a product.
12
Enzyme Inhibition and Bioapplications
Reversible inhibition is of short duration in the biological system because it depends on
substrate availability and/or rate of the catabolic clearance of the inhibitor (Figure 7).
E+S
E+I
K1
Ki
K-1
ES
EI + S
K2
1
Vo
E+P
Increases in inhibitor
concentration
1
Vmax
No product
Increases in
1
aKm
1
[S]
Fig. 7. The equation and the effect of the competitive inhibitor on the double reciprocal plot
of the substrate-reaction rate relationship.
Kinetically, the inhibitor (I) binds the free enzyme reversibly to form enzyme inhibitor
complex (EI) that is catalytically inactive and cannot bind the substrate. The competitive
inhibitor reduces the availability of free enzyme for the substrate binding. Thus, the Km of
the normal reaction is increased to a new Km (aKm) as a function of the inhibitor
concentration (expressed in the "a" factor - apparent Km in presence of the inhibitors), where
the substrate concentration at Vo = ½ Vmax is equal to aKm. The "a" can be calculated from the
change in the slope of the line at a given inhibitor concentration;
a = 1+
[I]
[E][I]
, w here, K I =
KI
[EI]
(1)
Therefore, competitive inhibitors do not affect the turnover number (active site catalysis per
unit time) or the efficiency of the enzyme because once enzyme is free, enzyme behaves
normally. The Michaelis-Menten equation for competitive inhibitors becomes
Vo =
Vmax [S]
aK m + [S]
(2)
Consequently, the double reciprocal form of the equation is also modified so as the line
1
slope becomes aK m and the intercept with y-Axis stays at
but the intercept with
Vmax
Vmax
the x-axis at -
1
aK m
will differ according to the concentration of the competitive inhibitor.
The later property is characteristic for competitive inhibitors.
Examples include the classical competitive inhibitory effect of malonic acid on succinate
dehydrogenase (SD) of the Krebs' cycle that reversibly dehydrogenates succinate into
fumarate. Other less potent competitive inhibitors of succinate dehydrogenase include;
oxalate, glutamate and oxaloacetate. The common molecular geometric feature of these
compounds is the presence of two negatively charged -COOH groups suggesting that the
active site of the flavoprotein SD has specifically positioned two positively charged binding
groups (Figure 8).
13
Enzyme Inhibition: Mechanisms and Scope
COO-
COO-
H2N CH
CH2
-
CH2
CH2
COO
COO-
C O
COOCOO Oxaloacetate
Glutamate
COO
Oxalate
-
CH2
-
-
COO
SD-FAD SD-FADH2
CH
Succinate
CH
dehydrogenase
COO
Fumarate
COO- +
CH2
SD
CH2
COO-+
Malonate COO
Succinate
Fig. 8. The substrate and different competitive inhibitors of succinate dehydrogenase (SD).
Methotrexate - competitive inhibitor of dihydrofolate reductase (DHFR) is another example.
The drug is used as anticancer antimetabolite chemotherapy particularly for pediatric
leukemia. It hinders the availability of tetrahydrofolate as a carrier for one-carbon moieties
important for anabolic pathways -particularly synthesis of purine nucleotides for DNA
replication (Figure 9).
CH3
NH2
N
N
O
N
H2N
C NH CH CH2 CH2 COO-
CH2 N
N
-
OOC
N Methotrexate
H
N
H2N
N
N
H
HN
O
N
H
H
H
C NH CH CH2 CH2 COO-
CH2 NH
Dihydrofolate
O
-
+
NADPH.H
OOC
HN
DHFR
NADP
+
N
R
H
O
Tetrahydrofolate
Fig. 9. The substrate and methotrexate as a competitive inhibitor for dihydrofolate
reductase.
Sulfanilamides - the simplest form of Sulfa drugs - were among earliest antibacterial
chemotherapeutic drugs classified as enzyme inhibitors. They are competitive inhibitors of
the bacterial folic acid synthesizing enzyme system from p-aminobenzoic acid. Bacterial
cannot absorb pre-made folate that is necessary to be synthesized de novo. Structural
similarity of sulfanilamide (and other sulfas derived from it) to p-aminobenzoic acid made
them competitive inhibitors to the enzyme (Figure 10).
SO2
H2 N
NH2
Sulfanilamide
Pteridine ring
N
H2 N
COOH
H2 N
N
p-Aminobenzoic acid
Glutamate
HN
N
O
CH2
C
NH
O
Folate
NH
CH
CH2
CH2
COO-
-
OOC
Fig. 10. The p-aminobenzoic acid substrate and sulfanilamide as a competitive inhibitor
during the bacterial folate synthesis.
14
Enzyme Inhibition and Bioapplications
Male erectile impotence was a major medical problem. Now a group of chemicals with
molecular structural similarity to cGMP is promising that competitively inhibit the cGMPphosphodiesterase-5. They include sildenafil citrate (Viagra; Figure 11), vardenafil (Levitra)
and tadalafil (Cialis). The inhibition of this enzyme that has a limited tissue distribution
including the penile cavernous tissue spares cGMP. Accumulation of cGMP leads to smooth
muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries, resulting in
increased inflow of blood and an erection.
O
O
O
N
O
N
HN
S
N
N
N
HN
N
N
H 2N
O
N
H
Sildenafil
cGMP
O
H
H
OH
O
O
H
P
O-
O
Fig. 11. The cGMP substrate and sildenafil a competitive inhibitor of the cGMPphosphodiesterase-5.
Another example of these substrate mimics competitive inhibitors are the peptide-based
protease inhibitors, a very successful class of antiretroviral drugs used to treat HIV, e.g.,
ritonavir that contains three peptide bonds (see Figure 12).
S
N
N
S
N
O
O
O
HN
HN
OH
HN
O
Fig. 12. The peptide-based competitive protease inhibitor ritonavir.
Reversible competitive inhibitors of acetylcholinesterase, such as edrophonium, physostigmine,
and neostigmine, are used in the treatment of myasthenia gravis and in anesthesia. The
carbamate pesticides are also examples of reversible acetylcholinesterase inhibitors.
15
Enzyme Inhibition: Mechanisms and Scope
4.2 Uncompetitive reversible inhibition
Uncompetitive inhibitor has no structural similarity to the substrate. It may bind the free
enzyme or enzyme substrate complex that exposes the inhibitor binding site (ESI). Its binding,
although away from the active site, causes structural distortion of the active and allosteric sites
of the complexed enzyme that inactivates the catalysis. This leads to a decrease in both Km and
Vmax. Increasing substrate towards a saturating concentration does not reverse this type of
inhibition and reversal requires special treatment, e.g., dialysis. This type of inhibition is also
encountered in multi-substrate enzymes, where the inhibitor competes with one substrate (S2)
to which it has some structural similarity and is uncompetitive for the other (S1). The reaction
without the inhibitor would be; E + S1  ES1 + S2  ES1S2  E + Ps and with uncompetitive
inhibitor becomes; E + S1  ES1 + I  ES1I (prevents S2 binding)  no product. It is a rare
type and the inhibitor may be the reaction product or a product analog.
Kinetically, uncompetitive inhibition modifies the Michaelis-Menten equation by (a') factor
that proportionates with the inhibitor concentration to be:
Vo =
V m ax [S]
K m + a'[S]
(3)
and in the double-reciprocal equation to be:
K
1
a'
1
=
+ m X
Vo Vmax Vmax [S]
while y-intercept is at
(4)
a' and x-intercept is at
a ' , whereas, the line slope stays K m .

Vmax
Vmax
Km
This gives a number of lines in the Lineweaver-Burk plot that are parallel to the normal line
with decreased 1/Vmax and –a'/Km proportional to concentrations of the uncompetitive
inhibitor. The later is characteristic to uncompetitive inhibition (Figure 13).
1
Vo
E+S
K1
K-1
ES
K2
Ki
I
ESI
E+P
Increases in inhibitor
concentration and
Decreases in a'/Vmax
a'
Vmax
No product
Decreases in
a'
Km
1
[S]
Fig. 13. The equation and the effect of the uncompetitive inhibitor on the double reciprocal
plot of the substrate-reaction rate relationship.
Uncompetitive reversible inhibition is rare, but may occur in multimeric enzymes. Examples
of uncompetitive reversible inhibitors include; inhibition of lactate dehydrogenase by
oxalate; inhibition of alkaline phosphatase (EC 3.1.3.1) by L-phenylalanine, and, inhibition of
the key regulatory heme synthetic enzyme; δ-aminolevulinate synthase and dehydratase
and heme synthetase by heavy metal ion, e.g., lead. Heavy metals, e.g., lead, form
mercaptides with -SH at the active site of the enzyme (2 R-SH + Pb  R-S-Pb-S-R + 2H).
16
Enzyme Inhibition and Bioapplications
Oxidizing agents, e.g., ferricyanide also oxidizes -SH into a disulfide linkage (2 R-SH  R-SS-R). Reversion here requires treatment with reducing agents and/or dialysis.
4.3 Mixed (noncompetitive) inhibition
The mixed type inhibitor does not have structural similarity to the substrate but it binds
both of the free enzyme and the enzyme-substrate complex. Thus, its binding manner is not
mutually exclusive with the substrate and the presence of a substrate has no influence on
the ability of a non-competitive inhibitor to bind an enzyme and vice versa. However, its
binding - although away from the active site - alters the conformation of the enzyme and
reduces its catalytic activity due to changes in the nature of the catalytic groups at the active
site. EI and ESI complexes are nonproductive and increasing substrate to a saturating
concentration does not reverse the inhibition leading to unaltered Km but reduced Vmax.
Reversal of the inhibition requires a special treatment, e.g., dialysis or pH adjustment. Some
classifications differentiate between non-competitive inhibition as defined above and mixed
inhibition in that the EIS-complex has residual enzymatic activity in the mixed inhibition.
Kinetically, mixed type inhibition causes changes in the Michaelis-Menten equation so as
Vo =
Vmax [S]
aK m + a'[S]
(5)
Mixed type inhibition - as the name imply - has a change in the denominator with Km
modified by factor (a) as in competitive inhibition, and [S] modified by factor (a') as in
uncompetitive inhibition. In the double reciprocal equation 6,
aK m
1
a'
1
=
+
X
Vo Vmax Vmax [S]
(6)
A line slope is aK m , and the intercept with y-axis is at a' and with x-axis is at
Vmax
Vmax
a' . This
aK m
results in progressive decreases in Vmax and progressive increases in Km proportional to the
increase in the mixed inhibitor concentration. The double reciprocal plot shows a number of
lines reflecting decreases in Vmax/increases in Km but their intercept is to the left of the yaxis. Mixed type inhibitor would be called non-competitive only if [a = a'], where, it will
only lower Vmax without affecting the Km (Figure 14).
1
Increases in inhibitor
concentration and
Decreases in a'/Vmax
Vo
E+S
I
K1
EI
S
K-1
ES
K2
Ki
I
E+P
ESI
a'
Vmax
No product
Increases in
a'
aKm
1
[S]
Fig. 14. The equation and the effect of the mixed type (noncompetitive) inhibitor on the
double reciprocal plot of substrate-reaction rate relationship.
17
Enzyme Inhibition: Mechanisms and Scope
Examples of noncompetitive inhibitors are mostly poisons because of the crucial role of the
targeted enzymes. Cyanide and azide inhibits enzymes with iron or copper as a
component of the active site or the prosthetic group, e.g., cytochrome c oxidase (EC
1.9.3.1). They include the inhibition of an enzyme by hydrogen ion at the acidic side and
by the hydroxyl ion at the alkaline side of its optimum pH. They also include inhibition
of; carbonic anhydrase by acetazolamide; cyclooxygenase by aspirin; and, fructose-1,6diphosphatase by AMP. Cyanide binds to the Fe3+ in the heme of the cytochrome aa3
component of cytochrome c oxidase and prevents electron transport to O2. Mitochondrial
respiration and energy production cease, and cell death rapidly occurs. The central
nervous system is the primary target for cyanide toxicity. Acute inhalation of high
concentrations of cyanide (e.g., smoke inhalation during a fire and automobile exhaust)
provokes a brief central nervous system stimulation rapidly followed by convulsion,
coma, and death. Acute exposure to lower amounts can cause lightheadedness,
breathlessness, dizziness, numbness, and headaches. Cyanide is present in the air as
hydrogen cyanide (HCN), in soil and water as cyanide salts (e.g., NaCN), and in foods as
cyanoglycosides. Comparison of the three types of the reversible enzyme inhibitors is
presented in Table 1.
In a special case, the mechanism of partially competitive inhibition is similar to that of noncompetitive, except that the EIS complex has catalytic activity, which may be lower or
even higher (partially competitive activation) than that of the enzyme-substrate (ES)
complex. This inhibition typically displays a lower Vmax, but an unaffected Km value. We
compare three main types of inhibitors in terms of reaction properties as shown in Table 1
and Figure 15.
Competitive inhibitor




The inhibitor binds the
catalytic/substrate
binding site.
It competes with
substrate for binding.
Inhibition is reversible
by increasing substrate
concentration.
Vmax constant, the
substrate concentration
has to be increased as
reflected on increased
Km.
Uncompetitive inhibitor



Substrate binding
exposes the inhibitor
binding site away from
the catalytic/substrate
binding site.
Increasing substrate
concentration does not
reverse the inhibition.
The inhibited reaction
rate parallel the normal
one as reflected on
decreased both Vmax and
Km.
Mixed
(noncompetitive inhibitor)




The inhibitor binds each
of the free enzyme and
the substrate-enzyme
complex away from the
catalytic/substrate
binding site.
Increasing substrate
concentration does not
reverse the inhibition.
Only Vmax is decreased
proportionately to
inhibitor concentration,
Km is unchanged since
increasing substrate
concentration is
ineffective.
Table 1. Comparison of the different types of reversible inhibition is shown in Table with a
quick view of mechanism in sketches as below.
18
Enzyme Inhibition and Bioapplications
Fig. 15. Sketch of three different enzyme inhibition by competitive, uncompetitive and
noncompetitive types are shown with illustration of enzyme-substrate or inhibitor binding,
kinetics and graphs.
In last decade, role of membrane receptors was explored in relation with enzyme inhibition.
Membrane receptors or transmembrane proteins bind with natural ligands such as
hormones, neurotransmitters in tissue membranes. Receptor-ligand binding modulates the
binding of drugs with enzyme. Such ligand binding behavior also influences the analysis of
competitive, uncompetitive and noncompetitive inhibition by biological effect of prodrugs
on enzymes. It usually involves a shape change in the receptor, a transmembrane protein,
which activates intracellular activities. The bound receptor usually does not directly express
biological activity, but initiates a cascade of events which leads to expression of intracellular
activity. However, occupied receptor actually expresses biological activity itself. For
example, the bound receptor can acquire enzymatic activity, or become an active ion
channel with similar competitive, noncompetitive behavior. Drugs targeted to membrane
receptors can have biological effects similar to the natural ligands, they are called agonists,
or conversely they may inhibit the biological activity of the receptor, they are called
antagonists [Jakobowski 2010a].
Enzyme Inhibition: Mechanisms and Scope
19
4.4 Agonist
An agonist or test drug or substrate is similar to natural ligand and binds with receptor to
produce a similar biological effect as the natural ligand. Agonist binds at the same binding
site in competition with natural ligand to show full or partial response. So, it is called partial
agonist. If receptor has a basal (or constitutive) activity in the absence of a bound ligand, it
is called inverse agonist. If either the natural ligand or an agonist binds to the receptor site,
the basal activity is increased. If an inverse agonist binds, the activity is decreased. Ro154513 and benzodiazepines (Valium) bind with the GABA receptor. As a result, GABA
receptor is "activated" to become a ion channel allowing the inward flow of Cl- into a neural
cell, inhibiting neuron activation. Ro15-4513 binds to the benzodiazepine site, which leads to
the opposite effect of valium, the inhibition of the receptor bound activity - a chloride
channel as shown in Figure 16.
Fig. 16. A sketch is shown for membrane receptor binding with ligand (agonist) acting like
as enzyme. Reproduced with permission [Jakobowski 2010a].
4.5 Antagonist
Antagonist or test inhibitor can inhibit the effects of the natural ligand (hormone,
neurotransmitter), agonist, partial agonist, and inverse agonists. We can think of them as
20
Enzyme Inhibition and Bioapplications
inhibitors of receptor activity behaving as competitive, noncompetitive and irreversible
antagonists as shown in Figure 17. For further details, readers are requested to read
advanced text book [Nelson et al. 2008, Dixon and Webb 1979]
Fig. 17. Sketch is shown for membrane receptor binding with ligand (acting as agonist) and
antagonist (acting as inhibitor) in competition with agonist to bind with enzyme.
Reproduced with permission [Jakobowski 2010a]
Enzyme Inhibition: Mechanisms and Scope
21
5. Inhibition by physiological modulators
5.1 Temperature of reaction
Some endothermic or exothermic chemical compounds change the temperature of reaction.
Enzyme reaction experiences inhibition at higher or lower than optimal physiological
temperature. For example, human body optimal temperature of human body is 37 oC. For
most of the enzyme reactions, enzyme activity usually increases at 0 to about 40-50 oC in the
absence of catalysts. As a general rule of thumb, reaction velocities double for each
increment of 10oC rise. At higher temperatures, the activity decreases dramatically as the
enzyme denatures as shown in Figure 18.
Fig. 18. Figure shows the effect of temperature change on the rate of enzyme reaction. Notice
the initial rise of rate of reaction and sudden fall near to optimal temperature 37-42 °C.
5.2 Hydrogen ion concentration or pH of reaction
Think of all the things that pH changes might affect. Many chemicals such as acids or
alkaline chemical compounds if mixed in enzyme reaction medium can change the pH. As a
result, reaction rate changes. It might




affect E in ways to alter the binding of S to E, which would affect Km
affect E in ways to alter the actual catalysis of bound S, which would affect kcat
affect E by globally changing the conformation of the protein
affect S by altering the protonation state of the substrate
The easiest assumption is that certain side chains necessary for catalysis must be in the correct
protonation state. Thus, some side chain, with an apparent pKa of around 6, must be
deprotonated for optimal activity of trypsin which shows an increase in enzyme activity with
the increase in range centered at pH 6. Which amino acid side chain would be a likely
candidate to participate in enzyme inhibition? It all depends on net charge on active group of
each amino acid in the active site chain. The pH of reaction thus depends on net pKa value of
amino acids and presence of acid or alkaline nature of substrate effects on enzyme kinetics by
formation of EH, ESH as shown in Figure 19. It can be modeled at the chemical and
mathematical level to calculate velocity(v), Vm(apparent) and Km(apparent) as shown in
Equations 7-9. Different enzymes show different behavior of enzyme catalyzed reactions such
22
Enzyme Inhibition and Bioapplications
as chymotrypsin, cholinesterase, papain, and papsin show distinct graphs (see Figure 20). For
further details, readers are requested to read text books [Nelson et al. 2008, Berg et al. 2011]
V=
(7)
Vm
1+H+/Kes1+Kes2 /H+
(8)
Km(1+H+/Ke1+Ke2 /H+)
1+H+/Kes1+Kes2 /H+
(9)
Vm app =
Km app =
Vm app S
Km app + S
Fig. 19. Chemical equations showing the mechanism of pH effects on enzyme catalyzed
reactions. Different mathematical equations 7-9 illustrate the modeling pH effects on
enzyme catalyzed reactions.
5.2.1 Three dimensional nature of enzyme-inhibitor complex at enzyme active site
The role of non-covalent interactions such as hydrogen bonding, hydrophobic interaction
and orientation of inhibitor and enzyme in an organized fashion was well described in
classic paper [Amtul et al., 2002]. 3D nature of enzyme reaction can be understood as
following. There are two sites on enzyme molecule: 1. at allosteric site, inhibitor binds with
enzyme, and 2. at active site, substrate binds with enzyme. However, substrate and inhibitor
interact with each other by non-covalent interactions of their chemical groups. Inhibitors
interact at allosteric site and known as ‘pharmacohores’. Presently, structure-based design
and testing, mechanistic biological approach is a state-of-art to develop new pharmacohores.
The non-covalent interactions determine the chemoselectivity of the substrate and enzymes
during formation of the ESI complex. In other words, ESI complex provides enzyme as a
platform to perform catalysis. 3D geometrical shape and topology of active site match with
orientation of chemical groups in substrate molecule that fit together in a ‘lock and key’
arrangement. Several possibilities happen to make enzyme-inhibitor complexes such as
bidentate, tri-, tetra- and polydentate, trigonal, pyramidal, tetrahedral, polyhedral charge
transfer complexes due to co-ordinate interactions between metallic co-factor with
hydrophilic groups on inhibitor(s). In this process, geometry of amino acid side chains at
allosteric site changes due to hydrogen bonding between amino acid residues. Suboptimal
Enzyme Inhibition: Mechanisms and Scope
23
Fig. 20. Graphs of different pH effects on enzyme catalyzed reactions as log Vm(app) and
Vm/Km(app) are shown on left. Different enzymes such as chymotrypsin, cholinesterase,
pepsin and papain are illustrated with different rates of enzyme reaction. Reproduced with
permission [Jakobowski 2010a]
interactions of metal-solvent, oxygen-water molecular bridge, free energy content loss,
subunit-subunit biophysical interactions as a result play a significant role in inhibitorenzyme complex formation and completion of enzyme catalysis.
For more details, readers are requested to read recent reference papers on 3D mechanistic
studies on enzymes. Specific example on urease is cited in chapter 11 in this book. Now
science is shifting to develop crystallized enzyme molecules, better structural-functional
relationship in enzyme catalysis and immobilized enzyme chips.
In following description, factors are discussed on different practical considerations that
influence the enzyme reaction rates, enzyme inhibition kinetics, % binding efficiency on
enzyme solid support with a glimpse of known theories and concepts on real-time, cheaper,
economic, user-friendly immobilized enzyme technology.
When actual and practical considerations are analyzed to work in enzyme reactor, the
scenario becomes complicated. Several factors such as inhibitor chemical state, substrate
structure, enzyme 3D conformation or peptide subunit interactions, physiological reaction
24
Enzyme Inhibition and Bioapplications
conditions in reactor and enzyme carrier supports also contribute in inhibition kinetics and
rates of reaction to form ES,ESI and P. Every year list of new factors grows in new enzyme
systems.
Author believes that more and more contributory factors introduced, will influence enzyme
reaction rate kinetics and more and more additive kinetic constants are introduced with new
variants to define the action of inhibitors on enzyme catalysis.
Other factors to keep in mind for new possibilities are:
1.
2.
3.
4.
5.
6.
7.
enzyme autoinhibition and enzyme molecular structural-functional factors affecting 3D
conformation of active site compatible with active groups of substrate or inhibitor
porosity and diffusion across the enzyme support material and availability of exposed
active sites to react
real-time recording the instant formation of ESI or ES or EP or EI on solid phase enzyme
support organic chip
sustrate-inhibitor interactions, % binding of active site with each additive
computer based semi-corrected or averaged calculations of kinetic constants of
inhibition kinetics
thermodynamic states of the enzyme reaction in reactor and fluctuating physiological
and physical states of substrate, inhibitor, enzyme complexes in reactor.
synergy of inhibitors, substrate, subunits in enzyme on active site
For all these factors and details, readers are expected to read advanced text books on
enzyme inhibition and enzyme engineering. Readers will experience a wide variation in the
scientific analysis of enzyme inhibition data in different enzyme reactors used in different
studies. High efficiency with desired results of enzyme inhibitors is the new challenges to
optimize reaction, scale-up, and phase out unwanted physiological factors from reaction. In
following section, these issues are addressed. Author believes that above mentioned
description is just iceberg from a large hidden treasure or unknown factors contributing
enzyme inhibition to give desired outcome.
6. Immobilized enzyme systems
In search of economic, efficient and practical enzyme platforms to test enzyme inhibitors,
new user-friendly immobilized enzyme technology is available now. It is based on principle
that an enzyme molecule is contained within confined space for the purpose of retaining
and re-using enzyme on solid medium in processing system or equipment. There are many
advantages of immobilized enzymes and methods of immobilization such as low cost,
suitability of reusable model system in membrane-bound enzymes in cell. However, some
disadvantages are expansive methods of adsorption or covalent bound or matrix trapping or
membrane trapping immobilization methods, low measurement of enzyme activity with
mass transfer limitations. For knowledge sake, the entrapment of enzyme molecules on
matrix, diffusion phenomenon and kinetics are important to understand. A brief description
is given for interested readers on classic concepts and scientific basis of porous or nonporous enzyme supports, theory of enzyme immobilization and efficiency of reaction
outcome. For more details of each aspect, readers are requested to read individual research
papers.
Enzyme Inhibition: Mechanisms and Scope
25
Matrix entrapment is done by mixing enzyme solution with polymer fluid in matrices such
as Ca-alginate, agar, polyacrylamide, collagen. Membrane entrapment is done by confining
enzyme solutions between semi-permeable membrane hollow fibers made of nylon,
cellulose, polysulfone, polyacrylate etc. Surface immobilization by adsorption is done by
attaching enzymes on stationary solids such as alumina, porous glass, cellulose, ionexchange resin, silica, ceramic, clay, starch etc. by physical forces keeping active sites intact.
Covalent bonding is done by enzyme retention on support surfaces by covalent binding
between functional groups such as amino, carboxylic, sulfhydryl, hydroxyl groups on the
enzyme and those on the support surface keeping enzyme active site(s) free (see Figure 21)
[Laider et al. 1980].
Fig. 21. Scheme of immobilization of enzyme is shown with chemical groups involved in
binding of enzyme on solid surface. Reproduced with permission from reference Lieder et
al.1980.
Diffusional limitations are observed to various degrees in all immobilized enzyme systems.
This occurs because substrate must diffuse from the bulk solution up to the surface of the
immobilized enzyme prior to reaction. The rate of diffusion relative to enzyme reaction rate
determines whether limitations on intrinsic enzyme kinetics is observed or not as shown in
Figures 22 [Laider et al.1980]. However, rate of diffusion across and within matrix is
determinant of immobilized enzyme reaction as shown in Figure 22 and 23.
In immobilized enzyme reaction, two major effects due to diffusion and product inhibition are
first observed by Lineweaber-Burk plots in classic study [Rees, 1984]. The diffusional effects
and product inhibition both influenced the shape of Lineweaver-Burk plot (see Figure 22). In
case of substrate inhibition effects binding of more than one substrate molecule(s) lead to
inhibition showing same type of curved Lineweaver-Burk plot as those observed for
diffusional limitation and product inhibition in immobilized enzymes. Combination of these
two effects lead to intermediate behavior, such as normal Michaelis-Menten kinetics as shown
26
Enzyme Inhibition and Bioapplications
in Figure 24, 25 by curves [Rees, 1984]. However, immobilized enzyme system also suffers
from both diffusion and product inhibition effects. As a consequence, it is important to
consider diffusion effects and product inhibition effects while extracting catalytic parameters
from kinetic data for immobilized enzyme systems. Use of non-porous support in enzyme
immobilization minimizes the diffusion effects to some extent.
k
k
p
i
E + S 
ES 
 EP 
E+P
Fig. 22. A sketch of porous matrix is shown (on left) and a scheme of substrate mass balance
Equation to calculate rate of immobilized enzyme reaction rs is shown (on right)
Fig. 23. A scheme of substrate mass balance is shown to calculate S with boundary
conditions.
Enzyme kinetics predicts the efficiency of reaction. Kinetics of immobilized enzymes
depends on conformational alterations within the enzyme due to the immobilization
procedure, or the presence and nature of the immobilization support. Immobilization can
greatly affect the stability of an enzyme such as any strain into the enzyme will inactivate
the enzymes under denaturing conditions (e.g. higher temperatures or extremes of pH). An
example of unstrained multipoint binding between the enzyme and the support to cause
substantial stabilization is illustrated in Figure 20. From mechanistic standpoint, a lesser
Enzyme Inhibition: Mechanisms and Scope
27
conformational change within the protein structure will initiate enzyme inactivation. As a
result, covalent immobilization processes involve an initial freely-reversible stage. Covalent
links may form, break and re-form till an unstrained covalently-linked structure is created.
However, additional stabilization is derived from maximum enzyme-support compatibility,
least enzyme molecule interactions, least proteolytic and microbiological attacks.
Fig. 24. Effect of one or more inhibitor molecules on enzyme kinetics and their inhibition
effect dependent on 1/So. Reproduced with permission from Rees et al. 1984.
Fig. 25. A scheme of immobilized enzyme action is shown on non-porous solid support.
Notice the dependence of Vm on available immobilized enzyme active sites (EL).
The kinetic constants (e.g. Km, Vmax) of immobilized enzymes may be altered by the process
of immobilization due to internal structural changes and restricted access to the active site.
Thus, the intrinsic specificity (k./Km) of such enzymes may well be changed relative to the
soluble enzyme. An example of trypsin is illustrated in Figure 21, where the freely soluble
enzyme hydrolyses fifteen peptide bonds in the protein pepsinogen but the immobilized
enzyme hydrolyses only ten. The apparent value of these kinetic parameters, when
determined experimentally, may differ from the intrinsic values. This fact may be due to
28
Enzyme Inhibition and Bioapplications
changes in the properties of the solution in the immediate vicinity of the immobilized
enzyme, or the effects of molecular diffusion within the local environment. The relationship
between these intrinsic and apparent parameters is shown below in Figure 26. Typically,
nonporous microenvironment consists of the internal solution plus part of the surrounding
solution which is influenced by the surface characteristics of the immobilized enzyme.
Partitioning of substances occurs between these two environments. Substrate molecule (S)
Intrinsic parameters of the soluble enzyme
Intrinsic parameters of the immobilized enzyme
Apparent parameters due to partition and diffusion
Fig. 26. A schematic cross-section of an immobilized enzyme particle (a) shows the
macroenvironment and microenvironment. Triangular dots represent the enzyme
molecules. Courtesy: Pangandai V. Pennirselvam, Ph.D UFRN, Lagoa Nova–Natal/RN
Campus Universitário. North East, Brazil.
Enzyme Inhibition: Mechanisms and Scope
29
diffuses through the surrounding layer (external transport) in order to reach the catalytic
surface and gets converted to product (P). In order for all immobilized enzyme to be
utilized, substrate must diffuse within the pores in the surface of the immobilized enzyme
particle (internal transport) [Pryciak 2008]. The degree of stabilization is determined by
strength of the gel, and hence the number of non-covalent interactions. As a result, intrinsic
parameters of enzyme result with specific apparent parameters dependent on partition and
diffusion as shown in Figure 27.



The porosity (e) of the particle can be expressed as ratio of the volume of solution
contained within the particle to the total volume of the particle. The tortuosity (t) is the
average ratio of the path length, via the pores, between any points within the particle to
their absolute distance apart.
The tortuosity, which is always greater than or equal to unity, depends on the pore
geometry. The diagram exaggerates dimensions for the purpose of clarity.
The concentration of the substrate at the surface of the particles [Sr] depends on radius
R or internal concentration [Si] at any smaller radius (r) is the lower value.
Fig. 27. Illustration of the use of multipoint interactions for the stabilization of enzymes.
(a) -------- activity of free un-derivatized chymotrypsin. (b) ….. activity of chymotrypsin
derivatized with acryloyl chloride. (c) -- -- -- activity of acryloyl chymotrypsin copolymerized
within a polymethacrylate gel. Up to 12 residues are covalently bound per enzyme molecule.
Lower derivatization leads to lower stabilization. (d) ----- activity of chymotrypsin noncovalently entrapped within a polymethacrylate gel. All reactions were performed at 60°C
using low molecular weight artificial substrates. The immobilized chymotrypsin preparations
showed stabilization of up to 100,000 fold, most of which is due to their multipoint nature
although the consequent prevention of autolytic loss of enzyme activity must be a significant
contributory factor. Reproduced with permission from Martinek et al, 1977a,b.
In general, the use of immobilized enzyme can be divided into two major categories of
applications: in biosensors and bioreactors. However, list is growing in the other fields of
ecological, environmental, agriculture, health, oceanic, space and earth sciences.
30
Enzyme Inhibition and Bioapplications
7. New developments in art of enzyme inhibition
Now a day, immobilized enzymes are used in industries and have value as medicinal and
industrial enzyme products. Good examples of industrial enzymes are amylase,
glucoamylase, trypsin, pepsin, rennet, glucose isomerase, penicillinase, glucose oxidase,
lipase, invertase, pectinase, cellulase in medicinal use. With emergence of new inhibitors in
the quest of drug discovery, several new inhibition mechanisms are expected in case of new
substrate analogues. New substrate–enzyme active site interactions are envisaged due to
different binding intricacies. Some examples of emerging concepts are outlined in following
description and readers are expected to read advanced literature on these applications.




Slow-tight inhibition: Slow-tight inhibition occurs when the initial enzyme-inhibitor
complex EI undergoes isomerizing conformational change to a more tightly binding
complex. However, the overall inhibition process is reversible. This manifests itself as
slowly increasing enzyme inhibition. Under these conditions, traditional MichaelisMenten kinetics gives a false value of a time-dependent Ki. The true value of Ki can be
obtained through more complex analysis of the on (kon) and off (koff) rate constants for
inhibitor association.
Substrate and product inhibition: Substrate and product inhibition is where either the
substrate or product of an enzyme reaction inhibits the enzyme's activity. This
inhibition may follow the competitive, uncompetitive or mixed patterns. In substrate
inhibition there is a progressive decrease in activity at high substrate concentrations.
This may indicate the existence of two substrate-binding sites in the enzyme. At low
substrate, the high-affinity site is occupied and normal kinetics is followed. However, at
higher concentrations, the second inhibitory site becomes occupied, inhibiting the
enzyme. Product inhibition is often a regulatory feature in metabolism and can also be a
form of negative feedback; see allosteric regulation [Pryciak 2008, Bashor 2008].
Antimetabolites: They are chemicals that interfere with the normal metabolism of
normal biochemical metabolite(s). This in most of case is due to their structural
similarity to such physiological substrates and therefore works as competitive enzyme
inhibitors. They include antifolates such as methotrexate, hydroxyurea and purine and
pyrimidine analogues. They are mainly used as cytotoxic anticancer drugs through
inhibiting DNA and RNA synthesis and cell division. An example of nitroimidazole is
described in detail on its metabolic effects at cellular level in this book [Sharma 2012a].
Antienzyme: Intestinal parasites, e.g., Ascaris, protect themselves from digestion by
expressing on their surface substances that are protein in nature which inhibit the action
of digestive enzymes, e.g., pepsin and trypsin. The blood plasma and extracellular
fluids are containing several types of protease inhibitors particularly important in
controlling the blood clot formation and dissolution and matrix and cytokine
homeostasis. Most of these inhibitors are peptides and several of them are also isolated
from raw egg white, potatoes, tomatoes and Soya bean and other plant sources. Most of
the natural peptide protease inhibitors are similar in structure to the amino acid
sequence of the peptide substrates of the enzyme. Designed peptide protease inhibitors
are important drugs, e.g., captopril that is a metalloprotease angiotensin-converting
enzyme peptide inhibitor. Inhibiting this enzyme prevent activation of angiotensin and
therefore prevent vasoconstriction to lower blood pressure. Crixivan is an antiretroviral aspartyl protease peptide inhibitor used in the treatment of Human
Enzyme Inhibition: Mechanisms and Scope




31
Immunodeficiency Virus (HIV)-induce acquired immunodeficiency syndrome (AIDS).
It inhibits the HIV protease that cleaves the large multidomain viral protein into active
enzyme subunits. Because these peptide inhibitors may not be specific, they have
several side-effects as drugs.
Antibodies against several nonfunctional plasma enzymes have clinical diagnostic
importance since they are longer living than the enzyme itself and hence reflect the
disease history better. In this respect, autoimmune antibodies are clinically important in
diagnosis of autoimmune diseases, e.g., anti-glutamic acid decarboxylase antibodies in
type 1 diabetes mellitus.
Biosensors: Light inhibits most enzyme activity although some enzymes, e.g., amylase
are activated by red or green light and also specific DNA repairing enzymes (e.g., UVspecific endonuclease) are activated by the blue and UV light. Ultraviolet rays and
ionizing radiations cause denaturation of most enzymes. Most enzymes contain
sulfhydryl (-SH) groups at their active sites which upon oxidation by oxidants and free
radicals by oxidants and free radicals inactivate the enzyme. Examples: Effect of
radiations, light and oxidants on the rate of the enzyme catalyzed reaction.
Other application of membrane bound redox enzymes constitutes them as a scaffolding
enzyme arrangement into systems for multi-step catalytic processes. The reconstruction
of portions of this redox catalytic machinery, interfaced to an electrical circuit leads to
novel biosensing devices or biosensors. An example of nitric oxide synthase enzyme is
cited in this book [Sharma, 2012b].
In EzNET® water purifying system, nitrate pollution is eliminated. Enzyme is
immobilized on “beads” with an electron-carrying dye as shown in Figure 28.
Reduction of nitrate to environmentally safe nitrogen gas is driven by a low voltage
direct current.
Fig. 28. EzNET® system shows immobilized enzyme on “beads” with an electron-carrying
dye. In this system, reduction of nitrate generates environmentally safe nitrogen gas driven
by a low voltage direct current. Source: The Nitrate Elimination Co., Inc. 2000.
32

Enzyme Inhibition and Bioapplications
In biolumescence detection for toxicity of HPV chemicals or drug development, 62 kDa
MW oxygenase (yellow green light emitted at 560 nm) enzyme gives 88 photon/cycle
light output proportional to [ATP] according to:
Luciferin + luciferase + ATP  luciferyl adenylate-luciferase + pyrophosphate
Luciferyl adenylate-luciferase + O2  Oxyluciferin + luciferase + AMP + light
Strong inhibition of luciferase by chloroform or HPV chemicals indicates the efficiency of
immobilized recombinant luciferase enzyme system as shown in Figure 20. Inhibition by
chloroform is much reduced in the mutant Luciferase compared to the wild type Luciferase
as shown in Figures 29, 30.
Source: Kim et al. AIChEngg Annual Meeting 2003, San Francisco, CA.
Fig. 29. A sketch of recombinant luciferase is shown illustrating the gene clone.

In the search for new therapeutics, the high throughput screening (HTS) of ligands for key
target proteins, enzymes represent the principal hit identification tool for early drug
discovery [Bartolini et al. 2009]. However, output depends on cost-based or amount-based
limitation of target availability, need of speed, automation and easy coupling of the
enzyme assay with separation systems (affinity chromatography of immobilized proteins)
and appropriate detectors. Good example is targeting in drug discovery represented by
enzyme inhibition mechanism in monolithic immobilized enzyme reactors (IMERs) to
represent different phases of the drug discovery pathway-starting with active compounds
(hit) identification, through drug development and lead optimization, early ADMET
(absorption, distribution, metabolism, excretion, toxicity) studies and quality control of
protein drugs. Some details are described in chapters in this book [Bartolini et al. 2005,
2007]. Interested readers are requested to read advanced text books on these
aspects. Different IMER have own requirements for optimal performances to show an
Enzyme Inhibition: Mechanisms and Scope
33
increased data output, reliability and stability to translate into cost reduction for potential
applications in pharmacy industry [Bartolini et al. 2005, 2007].
Source: Kim et al. AIChEngg Annual Meeting 2003, San Francisco, CA.
Fig. 30. Inhibition of luciferase activity by increasing the concentration of chloroform.
8. Softwares and computerization in enzyme inhibition kinetics
Recently softwares have popped up to visualize custom visual interface to see curve fits in
real-time, graph transforms, equations using kinetic data entry in terms of substrate, inhibitor,
activator, velocity, and standard deviation of the velocity. Data tables are directly generated
linked to the Fitting Panel of software. The data and results analysis is transferred in userfriendly lay-out, ANOVA window, % inhibition using Monte-Carlos fits, and receptor or
ligand binding calculator. For interested readers, VISUALENZYMICS 2010® is available for
statistical analysis for enzyme kinetics.[ http://www.softzymics.com/visualenzymics.htm].
9. Limitations and challenges
Above mentioned description on mechanism and applications shows a clear issue on need
of careful analysis for enzyme inhibition factors, presumptions of enzyme reaction, use of
new immobilized enzyme support and enzyme recording/monitoring methods. Challenge
is that most of times, basic presumptions do not hold true in enzyme reactors and addition
of new factors further complicate the calculation of reactor outcome. Most of the times,
computer based kinetic calculations average out outcome as less realistic with more chances
of variants. Other major challenge is that each time enzyme reactor outcome depends on
individual inhibitor and individual enzyme reactor at different times. It is less reproducible
34
Enzyme Inhibition and Bioapplications
and unpredictable because of synergy, interplay of known and unknown physical,
physiological, biological, molecular factors affecting reaction kinetics.
10. Impact of enzyme inhibition science in business
The major current and emerging therapeutic markets for enzyme inhibitors used in human
therapeutics are very high. New information is available on biochemistry for enzyme
inhibitors and classes of enzyme inhibiting products with broad current or potential
therapeutic applications in large markets. However, more than 100 enzyme inhibitors are
currently marketed and double than those are under development. A better understanding
of the emerging enzyme inhibitors on enzyme mechanism is main key. These include
selected indications for asthma and chronic obstructive pulmonary disease (COPD),
cardiovascular diseases, erectile dysfunction, gastrointestinal disorders, hepatitis B virus
infection, hepatitis C virus infection, herpesvirus infections, human immunodeficiency virus
(HIV)/acquired immune deficiency syndrome (AIDS) and rheumatoid arthritis and related
inflammatory diseases. Key information from the business literature and thorough enzyme
inhibition research is the basis of expert opinion on commercial potential and market sizes
from enzyme industry professionals. Since initial reports on chemical immobilization of
proteins and enzymes first appeared ∼30 years ago, immobilized proteins are now widely
used for the processing of products in industries from food business to environmental
control. In recent years, use of chemical immobilization was extended to immobilized
antibodies or antigens in bioaffinity chromatography. In coming years, it is speculated that
immobilization techniques of proteins and enzymes will have greater impact on point-ofcare medical and health business.
11. Conclusion
Enzyme inhibition is significant biological process to characterize the enzyme reaction,
extraction of catalysis parameters in bio-industry and bioengineering. Conceptual models of
inhibition define the interactions of substrate-enzyme or inhibitor-enzyme or both substrateenzyme-inhibitor in the moiety of active site. In recent years, application of enzymes and
enzyme inhibition science have gone in healthcare, pharmaceutical, bio-industries,
environment, and biochemical enzyme chip industries with great impact on healthcare and
medical business. Last decade has shown the measurement and accuracy of enzyme
detection up to the scale of picometer and enzyme industry is entering in the area of
picotechnology. Immobilized enzyme technology has given a new way of economic tools in
drug discovery and biosensor industry. Every year new enzyme inhibitors are discovered
useful as drugs but success still needs to minimize challenges.
12. Acknowledgements
Author acknowledges the suggestions of Dr Pagandai V. Pannirselvam, MTech, Ph.D at
Centro de Technologia, UFRN, Lagoa Nova–Natal/RN Campus Universitário. North East,
Brazil. Author contributed to explain intriguing issues on enzyme inhibition and
highlighted the need of better understanding on mechanism of inhibitors before applying
them in industries.
Enzyme Inhibition: Mechanisms and Scope
35
13. References
Amtul, Z. Atta, Ur. R., Siddiqui, R.A., Choudhary, M. I. (2002). Chemistry and mechanism of
urease inhibition. Current Medicinal Chemistry, Vol 9, pp 1323-1348.
Bartolini M., Cavrini V., Andrisano V. (2005) J. Chromatogr A, Choosing the right
chromatographic support in making a new acetylcholinesterase microimmobilized
enzyme reactor for drug discovery. Vol 1065, pp 135-144.
Bartolini M, Greig NH, Yu QS, Andrisano V. (2009) Immobilized butyrylcholinesterase in
the characterization of new inhibitors that could ease Alzheimer’s disease. J
Chromatogr A. Vol 1216(13), pp 2730-38.
Bartolini M., Cavrini V., Andrisano V. Characterization of reversible and irreversible
acetylcholinesterase inhibitors by means of an immobilized enzyme reactor. J.
Chromatogr. A (2007) Vol 1144, pp 102 –10.
Bartolini M, Andrisano V. (2009) Immobilized enzyme reactors into the drug discovery
process: The Alzheimer’s Disease case. Web Source:
http://www.farm.unipi.it/npcf3/pdf/BartoliniManuela.pdf
Bashor, C.J., Helman,N.C., Yan, S., Lim, W.A. Using Engineered Scaffold Interactions to
Reshape MAP Kinase Pathway Signaling Dynamics. Science.Vol 319 (5869), pp15391543
Berg, J.M., Tymoczko, J.L., Stryer, L. (2011) Biochemistry ISBN-13: 978-1429231152, Freeman
WH and Company.
Cleland, W.W.(1979) Substrate inhibition, Methods Enzymol. Vol 63, pp 500-513.
Dixon,M., Webb,E.C. (1979) Enzymes, 3rd ed., Academic Press, New York.
El-Metwally, T.H., El-Senosi, Y. (2010) Enzyme Inhibition. Medical Enzymology: Simplified
Approach.Chapter 6, Nova Publishers, NY. pp 57-77.
Jakbowski H. (2010a) Personal communication. Online study. Chapter 6- Transport and
Kinetics. C. Models of Enzyme Inhibition and D. More complicated Enzymes.
Internet source.
http://employees.csbsju.edu/hjakubowski/classes/ch331/transkinetics/olcompli
catedenzyme.html
--ibid- (2010b)
http://employees.csbsju.edu/hjakubowski/classes/ch331/transkinetics/olinhibiti
on.html
Laider, K., Bunting, P. (1980) The kinetics of immonbilized enzyme systems. Methods
Enzymol. Vol 64, pp 227-248.
Martinek, K., Klibanov, A.M., Goldmacher, V.S. & Berezin, I.V. (1977a) The principles of
enzyme stabilization 1. Increase in thermostability of enzymes covalently bound to
a complementary surface of a polymer support in a multipoint fashion. Biochimica
et Biophysica Acta, Vol 485, pp 1-12.
Martinek, K., Klibanov, A.M., Goldmacher, V.S., Tchernysheva, A.V., Mozhaev, V.V., Berezin,
I.V. & Glotov, B.O. (1977b) The principles of enzyme stabilization 2. Increase in the
thermostability of enzymes as a result of multipoint noncovalent interaction with a
polymeric support. Biochimica et Biophysica Acta Vol 485, pp 13-28.
Nelson, D.L., Cox, M.M. (2008) Lehninger Principles of Biochemistry. 5th Edition ISBN-13:
978071677108, Freeman W.H. and Company.
Pryciak, P. (2008) Customized Signaling Circuits. Science 319, pg 1489.
36
Enzyme Inhibition and Bioapplications
Rees, D.C. (1984) A general solution for the steady state kinetics of immobilized enzyme
systems. Bulletin of Mathematical Biology, Vol 46, 2,pp 229-234.
Sami, A.J., Shakoor, A.R.. (2011) Cellulase activity inhibition and growth retardation of
associated bacterial strains of Aulacophora foviecollis by two glycosylated
flavonoids isolated from Mangifera indica leaves. Journal of Medicinal Plants
Research (2011) Vol. 5(2), pp. 184-190.
Sharma,R. (1990) The effect of nitroimidazoles on isolated liver cell metabolism during
development of amoebic liver abscess. Dissertation submitted to Indian institute of
Technology, Delhi and CCS University.
Sharma, R. (2012a) Mechanisms of Hepatocellular Dysfunction and Regeneration: Enzyme
Inhibition by Nitroimidazole and Human Liver Regeneration. In: Enzyme Inhibition:
Concepts and Bioapplications. Chapter 7, InTech Web Publishers, Croatia. ISBN 979953-307-301-8.
Sharma, R. (2012b) Inhibition of Nitric Oxide Synthase Gene Expression: In Vivo Imaging
Approaches of Nitric Oxide with Multimodal Imaging. In: Enzyme Inhibition:
Concepts and Bioapplications. Chapter 8, InTech Web Publishers, Croatia. ISBN 979953-307-301-8.
Section 2
Applications of Enzyme Inhibition
2
Cytochrome P450 Enzyme
Inhibitors from Nature
Simone Badal, Mario Shields and Rupika Delgoda
University of the West Indies/
Natural Products Institute
Jamaica
1. Introduction
1.1 Cytochrome P450
Cytochrome P450 (CYP) is a heme containing enzyme superfamily that catalyzes the
oxidative biotransformation of lipophilic substrates to hydrophilic metabolites facilitating
their removal from cells. The CYPs were first recognized by Martin Klingenberg
(Klingenberg, 1958) while studying the spectrophotometric properties of pigments in a
microsomal fraction prepared from rat livers. When a diluted microsomal preparation was
reduced by sodium dithionite and exposed to carbon monoxide gas, a unique spectral
absorbance band with a maximum at 450nm appeared. The ferric ion in the resting heme,
binds easily with CO following reduction, and the complex’s maximal absorbance band,
unique amongst hemeproteins, serves as the signature of CYP enzymes.
CYPs are mostly located in the endoplasmic reticulum, and to some extent in mitochondrial
fractions of hepatic and extra-hepatic tissues. Even though these enzymes are ubiquitous in
the body (Table 1), of the 18 families in mammals identified, 11 are expressed in a typical
human liver (CYP1A2, CYP2A6, CYP2B6, CYP2C8/9/18/19, CYP2D6, CYP2E1, and
CYP3A4/5). In addition, five of these enzymes (CYPs 1A2, 2C9, 2C19, 2D6 and 3A4)
expressed at high levels in the liver demonstrate a broad substrate selectivity which
accounts for about 95% of drug metabolism (Nelson, 2009; Treasure, 2000).
The metabolism of a drug can be altered by another drug or foreign chemical and
such interactions can often be clinically significant. As a result, the FDA (Food and
Drug Administration) and other regulatory agencies such as the Department of Health
and Human Services (DHHS), Centers for Disease Control and Prevention (CDS)
and Hazard Analysis Critical Control Point (HACCP) among others expect information on
the relationship between each new drug to CYP enzymes (substrate, inhibitor and or
inducer) making these enzymes vital in the process of drug discovery. One of the major
concerns is avoiding drug interactions, an issue whose importance increases with the
aging of population (Guengerich, 2003) along with the increase in the practice of
polypharmacy.
40
Enzyme Inhibition and Bioapplications
Organ
CYPs detected
Nasal mucosa
2A6, 2A13, 2B6, 2C, 2J2, 3A
Trachea
2A6, 2A13, 2B6, 2S1
Lung
Oesophagus
1A1, 1A2, 1B1, 2A6, 2A13, 2B6, 2C8, 2D6, 2E1, 2F1, 2J2, 2S1,
3A4, 3A5, 4B1
1A1, 1A2, 2A, 2E1, 2J2, 3A5
Stomach
1A1, 1A2, 2C, 2J2, 2S1, 3A4
Small Intestine
1A1, 1B1, 2C9, 2C19, 2D6, 2E1,2J2, 2S1, 3A4, 3A5
Colon
1A1, 1A2, 1B1, 2J2, 3A4, 3A5
Table 1. Human cytochrome P450 genes expressed in different parts of the respiratory and
gastrointestinal tracts (adopted from Ding and Kaminsky, 2003).
1.2 Classification of CYP enzymes
All eukaryotic CYPs except fungal CYP55s are membrane bound; 18 mammalian CYP
enzyme structures are known and 15 of these are of human origin; [1A2, 2A6, 2A13, 2B4
rabbit, 2B6, 2C5 rabbit, 2C8, 2C9, 2D6, 2E1, 2R1, 3A4, 7A1, 8A1, 19A1, 24A1 rat, 46A1, 51A1,
(Nelson and Nebert, 2011)]. CYPs sharing >40% sequence identity are categorised within the
same family while those with >55% sequence identity are placed within the same subfamily.
The CYP superfamily members are named according to a nomenclature system that was
established in the mid-1980s (Nebert et al., 1987), however, the last comprehensive revision
was published in 1996 (Nelson et al., 1996).
CYP2 is the largest CYP450 family in mammals with 13 subfamilies and 16 genes in humans.
CYPs2C8, 2C9, 2C18 and 2C19 jointly metabolise more than 50 drugs whilst CYP2D6
metabolises more than 70 drugs (Meyer and Zanger, 1997). CYP3A is the most abundantly
expressed CYP450 gene in the human liver and gastrointestinal tract (Nelson, 1999) and is
known to metabolise more than 120 commonly prescribed pharmaceutical agents.
CYPs1Al and 1B1 are predominately expressed in extra-hepatic tissues (Guengerich and
Shimada, 1991; Shimada et al., 1992) while CYP1A2 is expressed primarily in the liver. As a
result, constitutive levels of CYP1A2 are much greater than those of CYPs1A1 and 1B1
(Shimada et al., 1992; Shimada et al., 1994b) whose levels are usually induced by PAHs. All 3
members of the CYP1 family are upregulated by halogenated and polycyclic aromatic
hydrocarbons such as those found in cigarette smoke and charred food.
1.3 Importance of CYP enzyme inhibition
1.3.1 Involvement in drug interactions
The metabolism of a drug can be altered by another drug or foreign chemical and such
interactions can often be clinically significant. The observed induction and inhibition of CYP
enzymes by various traditional remedies have led to the general acceptance that natural
therapies can have adverse effects. This is contrary to the popular beliefs in countries where
there is an active practice of ethnomedicine. Drug-herb interactions may involve
Cytochrome P450 Enzyme Inhibitors from Nature
41
competitive, noncompetitive, or uncompetitive inhibition of drug metabolizing enzymes or
enzyme induction by the phytopharmaceutical (Delgoda and Westlake, 2004).
Several epidemiological surveys including ones conducted by our laboratory (Delgoda et al.,
2004; Delgoda et al., 2010; Picking et al., 2011) have indicated high usage of herbal medicines
along with prescription medicines with low physician awareness. With over 80% of the
prescription medicine users also seeking some form of herbal remedy in Jamaica, the
chances of drug interactions rises and this prompted investigations into likely
pharamacokinetic, metabolism based interactions between the two types of medicines.
The CYP enzymes, responsible for the metabolism of over 90% of drugs in the market is
unsurprisingly associated with numerous metabolism related drug interactions
(Guengerich, 1997), including those of drugs and herbs (Ioannides, 2002; Delgoda and
Westlake, 2004). The inhibition of CYP3A4 by fucocoumarins found in grapefruit juice
leading to clinically observable toxicities with drugs and the induction of the same CYP3A4
enzyme by ingredients found in St. John’s wort leading to subtherapeutic interferences with
cycloporin provide suitable examples for the involvement of CYP enzymes in drug herb
interactions. While clinical studies provide the ultimate proof for relevant drug interactions,
in-vitro laboratory evaluations with CYP enzymes, has provided a convenient, economical
and useful starting point for screening those herbs that may ultimately cause clinically
observable drug interactions. Human liver microsomes, heterologously expressed enzymes
and hepatocytes although with limitations, have provided convenient means for such initial
assessements.
In this chapter, we describe for the first time, the initial inhibitory impact of four commonly
consumed infusions on six major CYP enzymes. Our findings support that the teas are
moderate to weak CYP inhibitors and so we postulate that they would unlikely result in
drug interactions.
1.3.2 CYP inhibition and its relation to chemoprevention
Approximately five decades of systematic drug discovery and development have
established a reliable collection of chemotherapeutic agents (Yarbro, 1992; Chabner, 1991).
These chemotherapeutic agents have assisted with numerous successes in the treatment and
management of human cancers (Chabner et al., 1991).
Chemoprevention is the ability of compounds to protect healthy tissues via the prevention,
inhibition or reversal of caricnogenesis. The inhibition of CYP1 enzymes is one such route
among others that include the induction of cell cycle arrest, the induction of phase II
enzymes and the inhibition of inflammatory. The CYP1 family has been linked with the
activation of pro-carcinogens which is facilitated by the regulation of the aryl hydrocarbon
receptor. As such research has shown that inhibiting CYP1 enzymes plays a key role in
protecting healthy cells from the harmful effects of activated carcinogens.
Among the polycyclic hydrocarbons that are activated into reactive metabolites by CYPs
1A1 and 1B1 is benzo-a-pyrene [BaP]. Metabolites from BaP include phenols, polyphenols,
quinines, epoxides and dihydrodiols. Among these dihydrodiols; (-)-benzo[a]pyrene-trans7,8-dihydrodiol (BPD) and (+)-anti-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide (anti-
42
Enzyme Inhibition and Bioapplications
BPDE) are carcinogenic, however the latter is the ultimate carcinogen as it has been shown
to bind DNA predominantly at the N2-position of guanine to produce primarily N2-guanine
lesions, benzo-a-pyrene 7,8-diol-9,10-epoxide-N2-deoxyguanosine (BPDE-N2-dG) adduct
(Osborn et al., 1976). It is proposed that BPDE-N2-dG is linked to the high frequency of p53
G→T transversions observed in lung cancer of smokers (Hainaut and Pfeifer, 2001; Pfeifer et
al., 2002). Further mutations in the p53 gene have also been found and these include
transversions, G→A and G→C (Shukla et al., 1997; Schiltz et al., 1999). Similar to the role of
CYP1A1 in the activation of BaP is that of the aromatic amines; amino-3-methylimidazo[4,5f]quinoline (IQ), 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8dimethylimidazo-[4,5-flquinoxaline (MeIQx). CYP1A2 plays an important role in the Noxidation of these aromatic amines which have been linked to colon and urothelium cancers
(Landi et al., 1999), thus highlighting the role of CYP1 enzymes in carcinogenic activation
and thus their potential as preventative targets. Fig.1 is a schematic representation of the
process of carcinogenesis at the cellular level.
Fig. 1. A schematic representation of carcinogenesis via the activation of CYP1 enzymes.
Upon the activation of the pro-carcinogens by the CYP1 enzymes, they have the ability to
bind to DNA, which can lead to mutations and then the formation of cancer cells.
One of the first reported chemoprotectants was disulfiram (Stoner et al., 1997) which inhibited
the action of dimethylhydrazine via the inhibition of CYP1 enzymes. Other chemopreventive
agents are discussed by Chang and others (Chang et al., 2002) who report that Ginseng
decreases the incidence of 7,12 dimethyldenz(a)anthracence (DMBA)-initiated tumorigenesis
in mice via the inhibition of CYPs1A1, 1A2 and 1B1. Also, the flavanoid, galangin was found
Cytochrome P450 Enzyme Inhibitors from Nature
43
to be an agonist of the aryl hydrocarbon receptor and consequently was responsible for an
increased level of CYP1A1 expression, however this effect was counteracted by its ability to
inhibit the enzyme directly and so is deemed an effective chemo-preventive agent (Ciolino and
Yeh, 1999). Resveratrol was also found to exhibit chemo-preventive properties via the
inhibition of CYP1A1 expression in vivo by preventing the binding of the AhR to promoter
sequences that regulate the CYP1A1 transcription and also by the direct potent inhibition of
CYPs1A1 and 1B1 (Ciolino et al., 1998; Chen et al., 2004).
1.4 CYP inhibition and its relation to chemoprevention
Bioactivity of isolates from the Jamaica plants, Amyris plumieri, Peperomia amplexicaulis,
Spathelia sorbifolia and Picrasma excelsa are reported in this chapter. Amyris plumieri is found
in the Caribbean, Central America and Venezuela and plants of this genus have been used
in folk medicine against skin irritation while isolates have been found to exhibit anticancer
and antimycobacterial properties (Fuente et al., 1991, Hartwell, 1968). Even though both
Peperomia amplexicaulis and Spathelia sorbifolia are not commonly consumed in Jamaica,
isolates from these plants have been shown to exhibit antiprotozoal, chemopreventive and
anti-cancer activity (Mota et al., 2009; Cassady et al., 1990) and previously examined for CYP
inhibitions (Badal et al., 2011; Shields et al., 2009) and overviewed in this chapter. Infusions
of the plant Picrasma excelsa, known as Jamaican bitterwood tea, are commonly consumed to
lower blood sugar levels in diabetics who are already on prescription medicines. All other
plants investigated in this chapter; Rhytidophyllum tomentosa, Psidium guajava, Symphytium
officinale, Momordica charantia are frequently consumed in the form of teas or the fruits of the
appropriate plants. We therefore investigated the inhibition properties of these teas against
a panel of CYP450 enzymes in order to assess the potential for drug interactions with comedicated pharmaceuticals.
2. Materials and methods
2.1 Chemicals
All CYP substrates and metabolites were purchased from Gentest Corporation (Woburn,
MA, U.S.A.). All other chemicals were purchased from Sigma-Aldrich (MO, U.S.A.).
2.2 CYP microsomes
Escherichia coli membranes expressing human CYP2D6, CYP3A4, CYP1A1, CYP1A2 and
each containing P450 reductase, were a gift from Dr. Mark Paine and Prof. Roland Wolfe
(University of Dundee, UK). CYP2C19 expressed in baculovirus-insect cells (supersomes)
were purchased from Gentest Corporation, Woburn, MA
2.3 Preparation of infusions from medicinal plants
The selection of the plants for screening and method of preparation were based on the
survey conducted by Delgoda et al (Delgoda et al., 2010). The teas were prepared by infusing
100ml of boiling deionized water per 1g of dried, finely ground material (leaf, bark or wood
chips), for 10 minutes. The resulting liquor was suctioned filtered through type 1 Watman
44
Enzyme Inhibition and Bioapplications
filter paper. A portion of the filtrate was then centrifuged at 13000 × g for 5 minutes to
remove suspended solids.
2.4 Separation of active ingredients from medicinal plants
Infusions were freeze dried and re-dissolved in water just prior to use, unless otherwise stated.
25µl infusions were loaded onto a microsorb C18 column (ID 4.6mm, 25cm, 5m) and separated
using the appropriate solvent systems using Varian Prostar HPLC system (Varian Inc. USA).
2.5 CYP inhibition assays
Routinely, appropriate volumes of potassium phosphate buffer (KPB), test inhibitor, CYP,
and the substrates were added to a NADPH regenerating mixture and made up to 400 µL,
and monitored fluorometrically on a continuous basis for 10mins as described elsewhere
(Shields, 2009), using CYP450 substrates,3-[2-(N,N-Diethyl-N-methylamino)ethyl]-7methoxy-4methylcoumarin (AMMC), 7-Benzyloxy-4-trifluoromethylcoumarin (BFC), as
substrates for CYP3A4 and CYP2D6 respectively and 7-ethoxy-3-cyanocoumarin (CEC) as
substrate for CYPs 1A1, 1A2, 2C19 and 2C9. In other instances (as specified in each case), a
96-well plate assay was employed as detailed in (Badal et al., 2011). Fluoroscence was
monitored using a Varian Cary Eclipse Fluorescence spectrophotometer.
Positive control experiments were conducted with varying concentrations of furafaylline
(≥98%) (0.5-10µM), quinidine (≥90%, 1-50nM) and ketoconazole (≥98%) (2-100nM) with
CYP1A2, CYP2D6 and CYP3A4 respectively.
2.6 Data analysis
IC50 and Ki values were determined by fitting the data in Sigma Plot (version 10.0) and
enzyme kinetics module, using non linear regression analysis. The data listed represent the
average values from three different determinations.
3. Results
3.1 Optimising experimental conditions
To verify the accuracy of experimental techniques employed to detect CYP inhibition, assays
with known inhibitors were carried out with furafylline (against CYP1A2), ketoconazole
(against CYP1A1, CYP1B1 and CYP3A4), (−)-N-3-benzyl-phenobarbital (NBPB, against
CYP2C19) and quinidine (against CYP2D6) and the obtained IC50 values (0.8±0.2, 0.04±0.01,
6.3±1.7, 0.06±0.01, 0.3±191 0.01, and 0.03±0.01μM respectively) compared well with
published values (0.99, b10, b10, 0.06, 0.25 and 0.01μM respectively; Shields, 2009; Badal et
al., 2011; Powrie, 2010; Stresser et al., 2004; Cali, 2003 and McLaughlin et al., 2008).
3.2 Natural products as CYP inhibitors
Several classes of natural products were examined in our laboratory for their inhibitory
properties towards CYP450 enzymes. Chromene amides (CAs) isolated from Amyris
plumieri, quassinoids isolated from Picrasma excelsa, anhydrosorbifolin isolated from
45
Cytochrome P450 Enzyme Inhibitors from Nature
Spathelia sorbifolia and chroman 6 isolated from Peperomia amplexicaulis. Structures for these
can be seen in Figs. 2.1, 2.2 and 2.3 and in addition obtained IC50s can be seen in Table 2.
Both CA1 and quassin exhibited the most potency against CYP1A1. Both Anhydrosorbifolin
and chroman 6 and CAs, 1, 2 and 3moderately (IC50 between 1 and 10μM) inhibited the
activities of CYP1 family.
NH 2
HO
tyram ine
chrom ene ring
H
N
acyl residue
O
O
R
R = CH 3
acetam ide
R = CH(CH 3 )2
2-m ethylpropanam ide or isobutanam ide
R = CH 2 CH 2 CH 3
CA1
CA3
n-butanam ide
R = CH 2 CH(CH 3 )2
CA2
CA4
3-methylbutanam ide
3-m ethyl-2-butenam ide or -dim ethylacrylam ide
R = CH=C(CH 3 ) 2
CA6
benzam ide
R =
CA5
Fig. 2.1. Chromene amides
O
O
O
O
H3CO
H3CO
H
H
O
H
Quassin
Fig. 2.2. Quassinoids
H
O
O
H
H
Neoquassin
OH
H
46
Enzyme Inhibition and Bioapplications
O
OH
OH
O
HO
O
O
5-Hydroxy-2,7-dimethyl-8-(3-methylbut-2-enyl)-2-(4-methyl-penta-1,3dienyl)-chroman-6-carboxylic acid
O
Anhydrosorbifolin
Fig. 2.3. Others
Compounds
CYP isoforms
1A1
CA1
CA2
CA3
CA4
Quassin
Neoquassin
Anhydrosorbifolin
Chroman 6
1.31 ± 0.42
Ki =0.37
1.63 ± 0.53
Ki=2.40
2.43 ± 0.62
Ki=1.39
14.39 ±
7.40
9.2
Ki=10.8
11.9
Ki=11.3
4.9
2.1
1A2
1B1
32.80 ± 15.36 ± 0.42
4.45
6.25 ±
37.04 ± 1.51
1.85
189.84 ± 179.30 ± 20.5
7.60
18.59 ± 18.14 ± 1.02
0.67
57.6
ND
2C9
2C19
2D6
3A4
nd
92.5
0.77 ±
0.39
1.09 ±
0.52
2.43 ±
0.28
2.55 ±
1.85
262.5
2.22 ±
0.69
359.88 ±
144.55
11.70 ±
5.40
84.40 ±
3.5
217.8
1.14±0.4
8
15.48±0.
45
122.93±5
.95
7.63±1.2
6
47.0
nd
nd
nd
85.3
ND
80.6
113.4
184.1
24.5
1.9
1.4
nd
nd
nd
nd
5.8
5.6
nd
nd
nd
nd
Table 2. Summary of IC50 and Ki values (µM) obtained from the interaction of isomers of
chromene amides, quassinoids along with anhydrosorbifolin and chroman 6 using
heterologously expressed CYP microsomes. ND: Not determined due to intrinsic
fluorescence and quenching/enhancement of the metabolite nd: not done
3.3 Herbal infusions with CYP inhibitors
Hot water infusions of five popular herbs; Rhytidophyllum tomentosa, Psidium guajava,
Symphytium officinale, Momordica charantia and Picrasma excelsa were characterized for impact
as shown in Fig.3 and calculated IC50 values on the activities of CYP enzymes are shown in
Table 3.
47
Cytochrome P450 Enzyme Inhibitors from Nature
80
P
P
P
P
P
3A
2D
1A
2C
1A
4
6
2
19
1
60
40
20
100
C
C
C
C
C
80
Y
Y
Y
Y
Y
P
P
P
P
P
3A4
2D6
1A2
2C19
1A1
60
40
20
%
in h ib itio n o f C Y P
100
Y
Y
Y
Y
Y
a c tiv ity
120
C
C
C
C
C
%
in h ib itio n o f C Y P
a c tiv ity
120
0
0
1
10
100
1
10
[te a ] ( g /m L )
100
[te a ] ( g /m L )
A
B
120
120
% in h ib itio n o f C Y P a c tiv ity
100
% in h ib itio n o f C Y P a c tiv ity
100
CYP1A1
CYP3A4
CYP2C19
80
60
40
CYP1A1
CYP1A2
CYP2D6
CYP2C19
80
60
40
20
20
0
0
10
100
1000
10
[tea] (g/mL)
C
100
1000
[tea] (g/mL)
D
Fig. 3. Inhibition of CYP activity by medicinal plant infusions.
3.4 Identification of active ingredients
Due to the potency displayed against the activities of CYP450 enzymes, Psidium guajava was
selected for further characterization. Preliminary separation of the freeze-dried extract of
Psidium guajava by reverse phase HPLC (see Fig.4) revealed several resolved peaks and LCMS analysis at the same time and the results are summarized in Table 3. Two peaks were
identified as quercetin and hyperin whose structures are shown in Fig.5 and these displayed
48
Enzyme Inhibition and Bioapplications
50% inhibition against the activity of CYP2D6 enzymes as shown in Fig.6. Previously
modelled active site of CYP1A1 with bound quassin is displayed in Fig.7 where key residues
in the enzyme are identified; Asp313, Thr11, Ser124, Phe123, Ile386 and Leu496 in the
interaction between quassin and neoquassin.
Inhibition of CYP activity by Rhytidophyllum tomentosa infusion (A); Psidium guajava (B);
Momordica charantia (C); and Symphytium officinale (D). Different volumes of reconstituted
freeze-dried infusion were added to the incubation mixture, along with the CYP isoform,
substrate, and 6GPDH, and monitored fluorometrically over time, as described in Materials
and Methods. Control enzyme activity (mean ± SEM) for CYP3A4, CYP1A1, CYP2D6,
CYP1A2, CYP2C19, and CYP2C9 was 0.147 ± 0.037, 0.907 ± 0.095, 0.005 ± 0.000, 1.45 ± 0.04,
0.054 ± 0.016, and 0.057 ± 0.004μM/min/pmol of CYP, respectively. Curves for CYP2D6 and
CYP1A2 in for Memordica charantia (C) and for CYP3A4 in by Symphytium officinale (D) were
not included because their IC50 values exceeded 200g/mL.
Isoform
IC50 (g/mL)
Rhytidophyllum
tomentosa
Psidium
guajava
Momordica
charantia
Symphytium
officinale
Picrasma
excelsa*
CYP1A1
10.2
4.9
137.1
24.0
15.0
CYP1A2
28.3
24.0
>200.0
40.3
19.1
CYP2D6
158.0
26.3
>200.0
127.9
>200
CYP2C19
93.8
23.3
91.0
172.7
199.9
CYP3A4
178.1
48.7
82.3
>200.0
122.8
*Values for Picrasma excelsa were obtained from Shields et al, 2008.
Table 3. Summary of IC50 values obtained for the extracts from Fig. 2
15.924
Norm.
10
11.727
12.271
10.068
10.525
10.795
20
18.341
30
17.093
40
15.139
13.817
50
19.397
17.472
60
0
8
10
12
14
Fig. 4. HPLC profile of Psidium guajava extract
16
18
20
22
m
49
Cytochrome P450 Enzyme Inhibitors from Nature
OH
OH
OH
OH
O
HO
O
HO
OH
O
O
OH
OH
OH
O
O
Quercetin
OH
HO
Hyperin
Fig. 5. Structures of quercetin and hyperin (quercetin-3-D-galactoside).
Fig. 6. HPLC profile of Psidium guajava (adapted from Shields et al., 2009).
OH
50
Enzyme Inhibition and Bioapplications
Fig. 7. Interaction of quassinoids with CYP1A1
4. Discussion
Cytochrome P450 enzymes have been of particular interest in the field of drug discovery
for numerous reasons including the involvement of these enzymes in the metabolism of
over 95% of the drugs on the market and the potential of drug-drug interaction through
metabolism. CYP1 family which is under the regulation of the aryl hydrocarbon receptor
have been extensively researched and implicated in drug resistance as well as
carcinogenesis. CYP1B1 in particular, found in elevated levels in cancer tissues such as
those in colon is thought to provide a novel pathway for drug discovery and optimisation
for cancer treatment. Inhibitors of the activities of CYP1 enzymes are now accepted as
potential chemoprotectants by preventing the activation of polycyclic aromatic
hydrocarbons such as benzo-a-pyrene. Catalysed by CYP1A1 and CYP1B1, metabolites of
this pro-carcinogen, (+)-anti-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide (antiBPDE) and that 3-hydroxybenzo [a]pyrene (3HBaP) have been shown to bind to DNA
predominantly at the N2-position of guanine to produce N2-guanine lesions, benzo-apyrene 7,8-diol-9,10-epoxide-N2-deoxyguanosine (BPDE-N2-dG) adduct (King et al., 1976).
Thus inhibitors of CYP1 enzymes hold the potential to prevent the formation of such
damaging precursors that initiate malignant cancers of the breast, colon, lung and
urothelium among others.
In this chapter we highlight the potential of a few natural products abundant in the
Caribbean: chromene amides isolated from Amyris plumieri, quassinoids isolated from
Picrasma excelsa, anhydrosorbifolin isolated from Spathelia sorbifolia and chroman 6 isolated
Cytochrome P450 Enzyme Inhibitors from Nature
51
from Peperomia amplexicaulis. We also report for the first time bioactive screening of CYP
enzymes in the presence of five aqueous infusions of popularly used herbs; Rhytidophyllum
tomentosa, Psidium guajava, Symphytium officinale, Momordica charantia and Picrasma excelsa.
Potent and selective inhibition of the CYP1 enzymes were found amongst the investigated
natural compounds in particular chromene amides and quassinoids. CA1 displayed potent
inhibition against the activity of CYP1A1, with a Ki of 0.37μM and an IC50 value of 1.31μM
while quassin inhibited this enzyme with an IC50 value of 9.2μM and Ki of 10.8μM with
selectivity extended throughout all CYP enzymes investigated except CYP2C19 for CA1.
The degree of potency and selectivity with which both compounds inhibited this enzyme
warrants further research as possible chemoprotectants. Previously known and studied
natural compounds deemed to possess chemoprotective properties due to their ability to
inhibit CYP1A1 include; quercetin (IC50=1.36μM, Leung et al., 2007), curcumin (IC50=20μM),
demethoxycurcumin (IC50=21μM), ε-viniferin (IC50=1μM), resveratrol (IC50=30μM), and
sanguinarine (Ki =2μM). Both test compounds compare well with these known
chemoprotectants and thus warrant further research.
Both CYPs 1A1 and 1A2 share approximately 70% similarity in amino acid and the
specificity with which inhibition targeted towards CYP1A1 activity is noticeable in the
compounds CA1, CA3 and quassin. As such we unlocked the interaction between quassin
and CYP1A1 in previous publication (Shields et al., 2009). One of the first active site models
for CYP1A1 was demonstrated with quasin and important residues were highlighted;
Asp313, Thr11, Ser124, Phe123, Ile386 and Leu496 as shown in Fig.7 as being critical for
binding quassinoids.
CYP1B1 has been drawing keen interest for novel and anticancer therapeutics. Findings of
the over-expression of CYP1B1 in many tumour tissues compared with normal surrounding
cells, have led to the search for pro-drugs reliant on CYP1B1 metabolism for the conversion
into cytotoxic therapeutics. Although the role of such over- expression is yet to be fully
understood, it has been linked with drug resistance and in the promotion of cell survival
(Martinez et al., 2008). The modification in the expression levels of CYP1B1 has been shown
to modulate tumour progression (Castro et al., 2008) and thus specific inhibitors are
expected to be of therapeutic/preventive benefit. Although the potency and the specificity
of the chromene amides examined this chapter against CYP1B1 is not particularly high,
structure-activity relations may guide towards chromene amides with putative
improvement. Anhydrosorbifolin and chroman 6 displayed the most potency against this
enzyme deeming further investigations worthwhile.
Little impact towards the CYP1 family was observed in the presence of CA2 which could
be due to the isopropyl group on this chromene amide, compared with CA1. Even though
the Ki against CYP1A1 was increased (to 2.63μM), the IC50 value remained more or less
the same as CA1 (1.63μM), displayed moderate to low potency against CYPs 1A2 and 1B1.
The structural change made to CA2 was more significant in binding CYP2D6 as the
inhibition dropped over a 100 times (IC50=360μM for CA2 vs 2μM for CA1). Hence, CA2
displayed characteristics of a useful molecular probe where all significant drug
metabolizing enzymes can be inhibited except for the activity of CYP2D6. Chain
elongation and the loss of branching in the n-propyl end unit to form CA3, have a
52
Enzyme Inhibition and Bioapplications
dramatic impact on the affinity to CYP1A2 and CYP1B1. The inhibition potency dropped
6 folds against CYP1A2 (from 32.8μM for CA1 to 189.8μM for CA3) and 10 folds against
CYP1B1 (from15.4μM to 179.3μM). Thus, CA3 appears to show increased selectivity in its
inhibition against CYP1A1. Exploring other side groups as well as shifting the position of
the existing side groups can be explored in hopes of increasing the potency of CAs
towards the activity of CYP1B1
There is a large consumption of natural medicines alone and concurrently with prescription
medicines in the Caribbean, as in many parts of the world. This is confirmed in a recent pilot
study done in which 80% of prescription medicine consumers also take natural remedies
(Delgoda et al., 2004; Picking et al., 2011). Also, adverse drug reactions (ADRs) accumulates
to over 2 million per year in the United States alone (Gurwitz et al., 2000), therefore, the
ability to predict drug interactions involving the CYP enzymes has become a key component
of the drug discovery process (Forti and Wahlstrom, 2008). Providing the FDA with the
metabolic profile of a new drug entity with CYP enzymes is the first step towards avoiding
adverse reactions (Delgoda and Westlake, 2004). Known drug –herb interactions with
clinical impact include grapefruit juice with felopidium, tricyclic anti depressants which is
medicated through CYP3A4 inhibition.
We report for the first time hot water infusions of Rhytidophyllum tomentosa, Psidium guajava,
Symphytium officinale, Momordica charantia in comparison with reported Picrasma excelsa
being tested against CYP enzymes activity. The greatest potency was observed in the
presence of Psidium guajava that inhibited CYP1A1 with an IC50 of 4.9µg/mL. All of the
infusions are commonly consumed, be it, the fruit or as teas. With the high levels of
polypharmacy practise that exist among Caribbean people and the world, having a
metabolic profile on teas or commonly consumed plants become of grave importance.
Results displayed in Table 3 point to is minimal risk through CYP mediated drug
interactions as the teas weakly inhibited the main drug metabolising enzymes. Because, the
activity of Psidium guajava towards CYP1A1 was the most potent we further evaluated the
identification of the active ingredients that could be responsible for the observed bioactivity
towards the CYP1A1 activity. The active ingredients were found to be quercetin and
hyperin (see Figs. 4 & 5), compounds known as inhibitors of CYP enzymes, where quercetin
isolated from St. John’s Wort has been previoulsy shown to inhibit activities of CYPs 1A2,
2C19 and CYP2D6 with IC50s of 3.87 µM, 6.23 µM and 20.99 µM respectively; while
hyperforin isolated from Ginko biloba shown to inhibit the activity of CYP3A4 with an IC50 of
4.30µM (Moltke et al., 2004; Zou et al., 2002). Inhibitions against the CYP enzymes appear to
be between moderate and weak which confirms data in our lab, thus evoking moderate
concern for potential interactions with co-medicated pharmaceuticals through CYP
mediated metabolism.
5. Conclusion
Review of compounds that have potent and selective inhibitory properties against the
activities of CYP1 family in particular CYPs 1A1 and 1B1 aid in identification of useful
chemoprotectors. CA1 and quassin warrant further research because they were both
potent against the activity of CYP1A1 while anhydrosorbifolin and chroma 6 targeted
Cytochrome P450 Enzyme Inhibitors from Nature
53
CYP1B1. In vitro and in silico models as demonstrated for the first time in this chapter are
useful tools in the process of drug development to approximate the risk of drug
interactions and in the process of target improvement of key enzymes in
chemoprevention. In particular, for herbal remedies they confer useful models for
evaluating the risks of adverse effects arising from interactions with co-administered
prescription medicines, for which drug-interaction information is not mandated by the
regulatory agencies. Once the initial risk is estimated, clinical drug-interaction studies can
be launched, thus providing a cost-effective sieving process prior to embarking on
rigorous and expensive investigations.
6. Acknowledgments
We are grateful to the International Foundation for Science (IFS), Sweden, the University of
the West Indies post graduate fund, the Forestry Conservation fund and the Luther Speare
Scholarship for financial support. We are also grateful to Professor Helen Jacobs for
provision of select natural products.
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3
Pharmacomodulation of Broad Spectrum
Matrix Metalloproteinase Inhibitors Towards
Regulation of Gelatinases
1CNRS
Janos
Sapi1,
Erika Bourguet1, William Hornebeck2,
Alain Jean-Paul Alix3 and Gautier Moroy4
UMR 6229, Institut de Chimie Moléculaire de Reims, IFR 53 Biomolécules, UFR
de Pharmacie, Université de Reims-Champagne-Ardenne
2CNRS UMR 6237, Laboratoire de Biochimie Médicale, MéDyc, IFR 53 Biomolécules,
UFR de Médecine, Université de Reims-Champagne-Ardenne
3Laboratoire de Spectroscopies et Structures Biomoléculaires (EA4303), IFR 53
Biomolécules, UFR Sciences, Université de Reims-Champagne-Ardenne
4INSERM UMR 973, Molécules thérapeutiques in silico (MTi), Université Paris Diderot
France
1. Introduction
Matrix metalloproteinases (MMP) constitute a family of 23 zinc- and calcium-dependent
endopeptidases that play pivotal functions in several physiological processes such as
embryogenesis, wound healing, vasculogenesis or stem cell mobilization (Nagase et al.,
2006). These enzymes were originally defined as matrix-degrading proteases, but a myriad
of other substrates have been discovered including cytokines, chemokines, growth factors
and their receptors, cell adhesion molecules and angiogenic factors. MMP were first
described to exert their degradative function extracellularly against matrix macromolecules
or at the pericellular microenvironment. Recently, MMP proved to cleave intracellular
substrates belonging to any subcellular compartments (Cauwe & Opdenakker, 2010).
Among them were notably apoptotic regulators, signal transducers, molecular chaperones
or transcriptional and translational regulators. Therefore, MMP can be considered as
proteases mainly controlling signaling events through processing cytokines, chemokines
and degrading matrix, liberating matrikines in the extracellular space, or in turn cleaving
enzymes involved in signal transduction inside the cells. MMP are regulated at distinct
levels including gene expression, compartmentalization, proenzyme activation, enzyme
inhibition, endocytosis, and finally substrate availability and affinity. MMP up-regulation
participates in tumor progression and metastasis, inflammatory disorders, cardiovascular
and autoimmune diseases (Hu et al., 2007; López-Otín & Matrisian, 2007; Mandal et al.,
2003; Murphy & Nagase, 2008).
All MMP are produced as proenzymes i.e. zymogen; enzyme latency is due to the
formation of a coordinated bond between the zinc atom in the active site and an amino
58
Enzyme Inhibition and Bioapplications
acid residue cysteine present in a consensus PRCGXPD sequence in MMP prodomain.
Proteolysis of the prodomain, action of reactive oxygen species (O2, NO) on the amino
acid residue cysteine and allosteric perturbation (Sela-Passwell et al., 2010) of the
prodomain can disrupt this Cys-Zn bond, a process named “cysteine switch” (Van Wart &
Birkedal-Hansen, 1990). In the active enzyme, the zinc atom is linked to three histidine
residues and a water molecule. A conserved glutamic acid residue (Glu) in the catalytic
domain HEBXHXBGBXHS polarizes the water molecule (Gomis-Rüth, 2009; Lovejoy et al.,
1994). This ligated water molecule attacks the carbonyl carbon of the scissile bond and
transfers a proton to Glu and then to the scissile nitrogen atom. Then Glu releases the
second proton from the water molecule to the scissile nitrogen atom and the peptide bond
is cleaved (Figure 1).
Glu
O
O
H
Glu
Glu
O
O
H
H
N
His
Zn2+
His
H
O
O
His
O
H
O
H
Glu
O
O
H
N
His
Zn2+
His
H
O
N
O
His
H
H
O
O
O
His
His
Zn2+
His
O
His
His
H N
H
Zn2+
His
Fig. 1. Mechanism of action of MMP (adapted from Lovejoy et al., 1994).
Historically, MMP were named according to their preferential action on matrix components:
collagenases (MMP-1, MMP-8, MMP-13), gelatinases (MMP-2, MMP-9), proteoglycanases or
stromelysins (MMP-3, MMP-10), macrophage elastase (MMP-12).
To date, a classification based on their domain organization is favoured: five of them are
secreted and the others are transmembrane proteins (MT-MMP) based on their structure
similarities (Table 1) (Egeblad & Werb, 2002).
MMP family is constituted by: a pre-domain involved in enzyme secretion, a pro-domain
including a cysteine residue interacting with the zinc atom in the catalytic domain that
maintains the inactive enzyme form. The catalytic domain is responsible of the MMP
activity. All MMP, except MMP-7, MMP-26 (28 kDa) and MMP-23 (56 kDa) possess a
hemopexin-like domain involved in the substrate interactions. The gelatinases (MMP-2 (72
kDa) and MMP-9 (92 kDa)) contain a gelatin-binding type II domain with three fibronectin
(Fn(II))-like repeats. MMP-11 (51 kDa) and MMP-28 (59 kDa) contain a furin motif for
recognition by furin-like serine proteinases. This motif is also present in MMP containing a
vitronectin-like domain (MMP-21 (70 kDa)) and membrane-type MMP (MT-MMP). In
addition, MT-MMP have a transmembrane domain and a short cytoplasmic domain or a
glycosylphosphatidylinositol anchored (MMP-17 (57 kDa) and MMP-25 (63 kDa)). Finally,
MMP-23 is a type II transmembrane MMP with a cysteine array and immunoglobulin-like
domain.
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
Designation
MMP-7
MMP-26
MMP-1
MMP-3
MMP-8
MMP-10
MMP-12
MMP-13
MMP-18
MMP-19
MMP-20
Main structure
Pre
P ro
c a ta ly tic
Zn
SH
P re
P ro
M in im a l-d o m a in
h em o pe xin
c a ta ly tic
Zn
S im ple h e m o p exin-d o m a in
S
S
MMP-14
MMP-15
MMP-16
MMP-24
MMP-17
MMP-25
P re
P ro
he m o pe x in
c a taly tic
FnFn Fn Zn
G el atin b in d ing
S
Pre
S
h em o pe x in
Pro
Fu
c atalyt ic
Zn
F u rin ac tiv ated
S
SH
Pre
P ro
h e m op exi n
Fu
c ata lytic
Zn
SH
Pr o
S
he m o pe xin
ca ta ly tic
Fu
Zn
G P I-a n ch o re d
SH
P re
P ro
Pr e
Pro
G PI
S
he m op e xin
Vn
Fu
c a ta lytic
Zn
V itro n e c tin -lik e
S
MMP-23
C y T rans m em b ra ne
Tm
S
P re
S
S
MMP-21
Matrilysin
Matrilysin-2
Collagenase-1
Stromelysin-1
Collagenase-2
Stromelysin-2
Metalloelastase
Collagenase-3
Collagenase-4
Enamelysin
SH
MMP-11
MMP-28
Name
SH
SH
MMP-2
MMP-9
59
Fu
cata ly tic
CA
Ig- lik e
S
T ype II trans m em brane
Gelatinase A
Gelatinase B
Stromelysin-3
Epilysin
MT1-MMP
MT2-MMP
MT3-MMP
MT5-MMP
MT4-MMP
MT6-MMP
XMMP
(Xenopus)
Femalysin
Table 1. MMP family. Pre: signal peptide, Pro: propeptide, Fn: fibronectin type II domain,
Fu: furin recognition site, Vn: vitronectin-like domain, TM: transmembrane domain,
Cy: cytoplasmic domain, GPI: glycosylphosphatidylinositol, CA: cysteine array, Ig-like:
immunoglobulin-like domain.
60
Enzyme Inhibition and Bioapplications
2. Structures and properties of gelatinases
2.1 Structure of gelatinases active sites
Gelatinases A (MMP-2) and B (MMP-9), as classified as both collagenases and elastases, are
involved to a great extent in pathologies affecting major elastic tissues (lung, arteries).
Among the MMP family members, gelatinases subclan, MMP-2 and MMP-9, do exhibit
several originalities that could be taken into account for the design of inhibitors.
MMP family proved to have a great homology of sequence and the zinc-containing catalytic
site is surrounded by subsite pockets named S1, S2, S3 for non-primed and S’1, S’2, S’3 for
the primed side (Terp et al., 2002).
The conserved amino acid residues in gelatinases active-site region (Cuniasse et al., 2005;
Kontogiorgis et al., 2005; Nicolotti et al., 2007; Rao, 2005) are given in Table 2.
The structural amino acid sequence of MMP is mainly similar except for the loop region (S’1
pocket), which displays different length and is composed of distinct amino acid
composition. The similarities are ordered as S’1 > S2 > S’3 > S1, S3 > S’2.
Selective and/or combined occupancy of these pockets were believed to direct selectivity of
inhibitor. More generally, it has been determined that such subsites display distinct potency
in driving selectivity in order S’1 > S2, S’3, S3 > S1 > S’2.
S’1 pocket located immediately to “the right” of the catalytic site differs notably in size and
shape among MMP and has been named specific pocket.
The S’1 pocket is deep, presenting an elongated and hydrophobic shape with an amino acid
residue Leu at position 197 for all MMP except MMP-1 and MMP-7. The variation of amino
acid residues among MMP, within this pocket, might be important. It adopts an extended
shape in both gelatinases, but S’1 pocket in MMP-2 forms a large channel nearly bottomless,
while it is slightly flexible in MMP-9 presenting a real pocket-like subsite.
The S’2 pocket is shallow, partly solvent-exposed and delimited on the top face by the
amino acid residue 158 and on the bottom face by the amino acid residue 218. Its size is
affected by the amino acid residues 162 (Asn), which is a Leu for both gelatinases, 163 (Val)
which is an Ala for both gelatinases and 164 (Leu).
The S’3 pocket is neutral and partly solvent-exposed and delimited by the amino acid
residues 222 (Leu) and 223 (Tyr). The size of this pocket is dependent on the amino acid
residue 193, which is a Tyr for both gelatinases (Table 2).
As a rule, the substrates bind weakly with the unprimed subsites; however, some
differences could be assigned between gelatinases.
The S1 pocket is shallow and hydrophobic. The same triad is pinpointed for MMP-2 and
MMP-9 (His166-Phe168-Tyr155 and His183-Phe185-Tyr172, respectively). The amino acid
residue 163 and to a lower extent the amino acid residue 155 influence the S1 subsite
interactions with an inhibitor. The amino acid residue 163 is a Leu for both gelatinases.
The S2 pocket is solvent-exposed and the amino acid residues 86, 169 and 210 are poorly
conserved in MMP family and affect the shape and the properties of this pocket. Its shape is
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
Table 2. Overview of favorable ligand properties and conserved domains of gelatinases
(adapted from Cuniasse et al., 2005; Nicolotti et al., 2007; Terp et al., 2002).
61
62
Enzyme Inhibition and Bioapplications
dependent on the amino acid residue Pro at the position 87, then the MMP-2 has its Phe87
leading to a small and hydrophobic pocket and MMP-2 interacts with positive charge
probes. When the Pro87 is lacking, another conformation was observed. The amino acid
residue at the position 169 is a Pro for MMP-9 and defines a large hydrophobic pocket.
Finally, the amino acid residue 210, Asn in MMP-9 and Glu in MMP-2, leads to a less
exposed pocket and notably plays a crucial role in enzyme selectivity. The S2 pocket is
important and differentiates both gelatinases.
The S3 pocket is composed of a hydrophobic cleft delimited by the amino acid residues 155
and 168. The pocket shape and size are influenced by the amino acid residue 155 which is a
Tyr and 168 which is a Phe for both gelatinases.
Although these subsites might direct enzyme specificities, interaction of gelatinases with
macromolecular substrates also relies on the presence of remote binding sites named
exosites that also notably act in driving enzyme action (Figure 2).
Proteolytic activation
by MT1MMP
Pre
Pro
catalytic
MMP-2
Gelatin binding domain
Collagen binding domain
(CBD)
Hinge domain
(465-475)
PEX-like
S
Fn FnFn Zn2+
S
His
His His
S
MMP-9
Hinge domain
(444-521)
Pre
CBD
Pro
catalytic
Dimerization
TIMP-2
TIMP-3
TIMP-4
v3
CC (MCP-3)
CXC (SDF-1)
Proteoglycan
Fibrinogen...
OG
PEX-like
S
Fn Fn Fn Zn2+
His
His His
Degradation of substrates
Degradation of substrates
Collagen
Elastin
Gelatin
Fibrillin
Fibrinogen
Aggrecan
Vitronectin
Fibronectin
Laminine
Tenascin-C...
Proteolytic activation
by MMP-3
Collagen
Elastin
Gelatin
Fibrillin
Fibrinogen
Aggrecan
Vitronectin
Laminine
Entactin...
S
S
OG OG OG
Homodimerization
TIMP-1
Ku
CD44
Proteoglycan...
Fig. 2. Functions of gelatinases domains.
2.2 Biological properties of gelatinases
Gelatinases are most often associated to the cell plasma membrane of normal or transformed
cells, thus targeting the proteolytic activity of invasive cells. Enzyme tethering to cell
periphery requires the carboxy terminal hemopexin-like domain, designated as PEX of 200
amino acid residues on average forming a four bladed -propeller structure.
In pro-MMP-2, the PEX domain interacts with the C-terminal domain of TIMP-2 (Tissue
Inhibitors of Matrix MetalloProteinase-2 (Brew & Nagase, 2010)) which allowed the complex
to tether to plasma membrane through interaction of the N-terminal part of the inhibitor to a
MT1-MMP homodimer: one molecule of MT1-MMP acting as a docking molecule, the other
catalyzing pro-MMP-2 activation (Itoh et al., 2006, 2011; Sato & Takino, 2010).
Of note, this PEX domain also reacts with TIMP-3 and TIMP-4 but no MMP-2 activation was
noted in such case.
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
63
It needs to be emphasized that interaction of PEX domain in MMP-2 with MT3-MMP also
leads to enzyme activation that can be enhanced by the prior binding of pro-MMP-2 to
chondroitin-4 sulfate chains-containing proteoglycan. PEX domain of MMP-2 was also
reported to bind to v3, CC chemokine as monocyte chemoattractant protein-3 (MCP-3),
CXC chemokine as stromal cell derived factor-1 (SDF-1) and fibrinogen.
Such PEX domain is also important in driving the pro-MMP-9 activation; in human
neutrophils formation of pro-MMP-9-lipocalin complex favours enzyme activation by
kallikrein. It is also important for localizing the enzyme at the cell periphery through
interaction with low density lipoprotein-receptor related protein (LRP), CD-91 or different
isomer forms of CD-44, protein Ku, and is involved in the formation of covalent complexes
with proteoglycans (Malla et al., 2008; Monferran et al., 2004).
An important property of this carboxy terminal domain, in association with an
unstructured, hydrophilic and flexible long-O-glycosylated domain (OG) in pro-MMP-9
which contains 11 repeats of the sequence T/SXXP (Figure 2), relies on its ability to catalyze
the intracellular formation of enzyme dimers (Van den Steen et al., 2001). Importantly, the
dimer form of pro-MMP-9 is more resistant to MMP-3 activation.
Gelatinases also appear unique in MMP family in exhibiting fibronectin type II domains
which are also designated as collagen binding domains (CBD) (Shipley et al., 1996). Indeed,
deletion of these domains in both enzymes led to protease devoided of collagen(s)gelatin(s)- or elastin-degrading capacity (Allan et al., 1995). Recent data also indicated that
OG could also mediate MMP-9 gelatin interaction by allowing the independent movement
of enzyme terminal domain (Vandooren et al., 2011).
3. Design of MMP inhibitors (MMPI)
Up to now, a myriad of MMPI has already been synthesized (Table 3).
The most important studies focus on the combinations of diverse structural modifications;
three classes of compounds have been developed: combined inhibitors, right hand side and
left hand side inhibitors based on the scissile bond in the catalytic site (Skiles et al., 2004;
Whittaker et al., 1999). Some of them have been used as potential therapeutic agents to limit
tumor progression. Instead of using MMP inhibitors as therapeutic treatment, they might be
also useful as preventive drugs or as biomarkers in early stage of cancer. Up to now, most of
the clinical trials in cancer were rather disappointing (Abbenante & Fairlie, 2005; Dormán et
al., 2010; Fingleton, 2007; Gialeli et al. 2011).
The first generation of MMPI was based on peptidomimetic skeleton containing a succinic
acid motif and a hydroxamic acid as zinc binding group (ZBG) (batimastat®, marimastat®,
solimastat®, galardin®, trocade®). Hydroxamic acid is a bidentate chelator, but it has also a
good binding affinity to other ions (Cu2+, Fe2+ and Ni2+). They are broad spectrum inhibitors
and led to musculoskeletal syndrome side effect mainly due to the presence of hydroxamic
acid.
Prinomastat® contains a ring embedded sulfo-succinic acid motif, which increases oral
availability and water solubility. Nevertheless, clinical trials have been discontinued after
phase III for musculoskeletal toxicity and poor survival rate.
64
Enzyme Inhibition and Bioapplications
Tanomastat® has a thioether function increasing the oral activity and selectivity for MMP-2,
MMP-3 and MMP-9. Unfortunately, clinical trials are discontinued for haematological
toxicity and poor survival rate.
MMP
Inhibitors
Indication
Clinical trials
development
Batimastat®
(BB-94)
British Biotech
Cancer
Broad spectrum
(MMP-1, -2, -3, -7, -9)
Phase I
(Discontinued)
Structure
O
HO
O
H
N
N
H
N
H
O
S
S
Marimastat®
(BB-2516)
British Biotech
Solimastat®
(BB-3644)
British Biotech
Cancer
Broad spectrum
(MMP-1, -2, -7, -9)
Phase III
(Discontinued)
O
HO
Cancer
Phase I
Broad spectrum (MMP(Discontinued)
1, -7)
Galardin®
(GM-6001)
Glycomed
Eye disease, COPD
Broad spectrum
(MMP-1, -2, -9, -12)
Prinomastat®
(AG-3340)
Agouron
Cancer
Macular degeneration
Broad spectrum
(MMP-2, -3, -7, -9, -13)
OH
N
H
HO
N
H
O
O
Phase I
(Discontinued)
O
H
N
O
N
H
O
O
HO
O
H
N
N
H
O
H
N
N
H
N
N
H
O
N
H
Trocade®
(Ro32-3555)
Roche
Rheumatoid arthritis
(MMP-1, -8, -13)
S
Phase III
(Discontinued)
HO
H
N
N
O O
S
O
O
Phase III
(Discontinued)
O
HO
N
N
H
O
O
N
O
N
H
Tanomastat®
(BAY 12-9566)
Bayer
Cancer
Arthritis
(MMP-2, -3, -8, -9)
Phase II
Phase III
(Discontinued)
Rebimastat®
(BMS-275291)
Bristol-Myers
Squibb
Cancer
Broad spectrum
(MMP-1, -2, -7, -9, -14)
Phase III
(Discontinued)
Metastat®
(CMT-3)
Collagenex
Cancer
(MMP-2, -9)
O
OH
S
Cl
O
N
O
N
O
SH
O
Table 3. Main MMPI in clinical development.
O
H
N
N
H
OH
OH
O
N
H
O
O
O
Phase II
NH2
H
H
OH
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
65
The thiol zinc binding group of rebimastat® is a weak monodentate ligand and the
musculoskeletal toxicity and its poor response led clinicians to stop treatment.
Finally, metastat® is a second generation of tetracyclines still in phase II clinical trials (Acne,
AIDS-related Kaposi’s sarcoma and mainly used in cancer). The only detected side effect is
its photosensibility. This inhibitor is selective of MMP-2 and MMP-9 and crosses the bloodbrain barrier. Actually, it is the most promising MMPI.
These failures are mainly due to their broad spectrum MMP inhibitory activity, the
similarity of their active sites with those of other metalloproteinases (ADAM, ADAMT…),
the poor selectivity of the chelating group, the administration of MMPI in late disease stage,
their poor pharmacokinetics, unavoidable side effects (musculoskeletal pain), toxicity and
limited oral bioavailability.
However, these efforts led to pinpoint the importance of MMPI selectivity and allowed the
identification of MMP as target and anti-target in various diseases progression (Overall &
Kleifeld, 2006). Certain MMP are both targets and anti-targets depending on the stage of the
disease.
4. Galardin® pharmacomodulation as a tool for designing specific
gelatinases inhibitors
For a decade, we have been involved in the pharmacomodulation of galardin®, a powerful
broad spectrum MMPI (Figure 3).
S '1
Zn
O
HO
A n ch or to o th e r ta r g e t
2+
H
N
N
H
O
N
H
O
Me
G a la rd in ®
Fig. 3.
P2
ZBG
S e le c tiv ity
N
H
Galardin® and
S '3
S '2
P '1 H
N
O
N
H
O
P '2
P '3
N
H
A na lo g ue s
analogues.
By the beginning of 1994, galardin® was in phase I clinical trials for age related macular
degeneration (ARMD) and as chronic obstructive pulmonary disease (COPD) by Glycomed.
In order to increase selectivity, the synthesis of analogues of galardin®, has been achieved.
The modifications have been focused on the P’1, P’2, P’3 groups and ZBG.
4.1 Influence of the S’1 subsite: Modulation of galardin® in gelatinases inhibition
Our first experiments started with the insertion of one unsaturation in P’1 position to
increase the hydrophobicity of the new compounds and to study the effect of substitution on
S’1 pocket specificity (Figure 4). For this purpose, replacement of isobutyl group by an
isobutylidene group of E geometry enhanced by 100-fold MMP-2 selectivity versus MMP-3
66
Enzyme Inhibition and Bioapplications
(IC50 = 1.3 and 179 nM, respectively) (Marcq et al., 2003). The double bond geometry was
found important for potency and selectivity as shown with the equimolar E/Z mixture
which displayed lower activity.
Pursuing these pharmacomodulations aiming at better MMP-2 selectivities, we planned to
increase hydrophobicity and rigidity with the dehydro and didehydro analogues which
were synthesized (analogue 2a-d and 3a-h).
P'1
O
HO
N
H
H
N
O
N
H
O
1
O
Me
H
N
R
O
N
H
O
Me
N
H
N
H
2a-d R = -NH-OH
P'1 = -(CH2)5-CH3, -(CH2)2-Ph,
-CH=CH-Me, -CH=CH-Ph
3a-h R = -OH
P'1 = -(CH2)n-CH3
Fig. 4. Pharmacomodulation of the P’1 group.
Introduction of either one or two unsaturations decreased their potent MMP inhibitory
activity as compared to parent molecule over all MMP (Moroy et al., 2007). However, the
presence of a phenyl group at the end of alkyl chain (2b and 2d) led to inhibitors with a
good activity and selectivity for MMP-9 (IC50 = 38 and 45 nM, respectively).
In parallel, C7 long alkyl chain containing galardin® 2a displayed a MMP-2 selectivity
comparing to MMP-9 (IC50 = 123 nM) (Table 4).
Compounds
Galardin®
1
2a
2b
2c
2d
3a
3b
3c
3d
3e
3f
3g
3h
ZBG
-NH-OH
-NH-OH
-NH-OH
-NH-OH
-NH-OH
-NH-OH
-OH
-OH
-OH
-OH
-OH
-OH
-OH
-OH
P’1
-iPr
-(CH2)5-Me
-(CH2)2-Ph
-CH=CH-Me
-CH=CH-Ph
-(CH2)6-Me
-(CH2)7-Me
-(CH2)8-Me
-(CH2)9-Me
-(CH2)10-Me
-(CH2)11-Me
-(CH2)14-Me
-(CH2)18-Me
MMP-1
1.5
18
32000
9130
984
1240
> 105
> 105
> 105
> 105
> 105
> 105
> 105
> 105
MMP-2
1.1
9.2
123
280
78500
120
7570
458
247
249
351
655
762
> 105
MMP-9
0.5
17
> 104
45
974
38
838
241
173
450
211
673
582
> 105
MMP-14
13.4
10
2660
53100
913
2490
> 105
> 105
> 105
> 105
> 105
> 105
> 105
> 105
Table 4. Influence of the P’1 chain length on the selectivity and potency of MMP inhibitors.
IC50 values are expressed in nM.
AutoDock 4.0 program (Huey et al., 2007; Morris et al., 1998) was used to perform the
computational molecular docking. AutoDockTools package was employed to prepare the
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
67
input files necessary to the docking procedures and to analyze the docking results. The
figures have been done using Pymol program (DeLano, W. L. 2002. PyMol Molecular
Graphics System, Palo Alto, CA. http://www.pymol.org).
Molecular modelling experiments confirmed the importance of the insertion of the alkyl
chains in the S’1 pocket, supporting the observed biological data for 2a (Figure 5).
Fig. 5. Complex between MMP-2 and analogue 2a. The MMP-2 secondary structure was
represented in cartoon. The analogue 2a is shown with sticks in which the C atoms are
colored in magenta. Zn atom is displayed as a grey sphere.
The S’1 pocket of MMP-2 is sufficiently deep to accommodate the long alkyl chain. On the
contrary, the S’1 pocket of MMP-9 at the end of the tunnel is restrained, like a funnel by the
amino acid residues Glu233, Arg241, Thr246 and Pro247. The S’1 pocket of MMP-9 is large
enough to accept a phenyl group at the entrance of the pocket such as compounds 2b and
2d.
In order to increase hydrophobicity, aiming at a better MMP-2 selectivity, the alkyl chain
was elongated with n = 8 to 20 carbon atoms as described in the literature (Levy et al., 1998;
Miller et al., 1997; Whittaker et al., 1999). Using galardin® as template, the activity and
selectivity were not really increased with the length of the linear chain. Batimastat®
inhibitors increase activity for MMP-2 with C8 long alkyl chain (IC50 = 0.6 nM) and a C12
analogue displays a MMP-2 selectivity comparing to MMP-1 (IC50 = 1 and 50000 nM,
respectively). Finally, a good activity for MMP-2 is obtained for C9 long chain marimastat®
analogues (IC50 < 0.15 nM) and a maximal selectivity occurs with a C16 for MMP-2 versus
MMP-1 (IC50 = 0.6 and 5000 nM, respectively).
68
Enzyme Inhibition and Bioapplications
Nevertheless, in our case no better IC50 were found for MMP-2 with increasing chain length
when an unsaturation was incorporated. However, a good selectivity for MMP-2 versus
MMP-1 and MMP-14 is observed. Also, carboxylic derivatives 3c and 3e displayed a good
selectivity for MMP-9 versus MMP-1 and MMP-14.
As stated previously, S’1 pocket is generally quite large in all MMP. However, the amino
acid residue Arg214 redefines the bottom of the pocket in MMP-1 leading to a small and
restricted S’1 pocket. The amino acid residue Arg214 can be flexible but in most cases, a
large P’1 group inhibitor is expected to bind weakly to MMP-1. Thus, it is not surprising, to
find that most of the MMP-1 inhibitors have relatively small P’1 groups.
MMP-14 appears as one key-proteolytic enzyme to promote cancer invasion and metastasis
(Hernandez-Barrantes et al., 2002). Up to now, only one pentacyclic sterol sulphate MMP-14
inhibitor was described to display MMP-14 selectivity while it exhibits only low potency
(Fujita et al., 2001).
S’1 pocket of MMP-14 was found to be two amino acid residues longer than those of
gelatinases. The amino acid residue Met237 allows favourable interactions with the
hydrophobic substituents at the bottom of the pocket, but unfavourable interaction with the
positively charged substituents.
Therefore, lack of inhibition of this enzyme by long alkyl chain (Table 4) is rather surprising.
The unsaturation might disturb the entrance of the S’1 pocket, but that is purely speculative.
Finally, it seems to be more difficult to fit MMP-14 pockets, perhaps in keeping with its
transmembrane localization and domain structure.
4.2 Influence of the S’2 subsite on gelatinases inhibition
Introduction of an alkyl chain in the P’2 position of the indole ring leads to lower gelatinase
inhibitory activity. Nevertheless, the selectivity for MMP-2 was pinpointed (Marcq et al.,
2003).
On the contrary, the introduction of a phenyl group at the P’2 position (analogue 4)
enhanced the selectivity towards MMP-2 maintaining a high potency (IC50 = 0.092 nM). To
the best of our knowledge, the large and solvent-exposed S’2 pocket could accommodate
large and hydrophobic groups (LeDour et al., 2008). A good activity was found for MMP-1
and MMP-14 (IC50 = 0.244 and 0.601 nM, respectively).
4.3 Influence of the unprimed subsites on gelatinases inhibition
It is well documented that the hydroxamic acid is one of the most powerful ZBG, but its
toxicity and low bioavailability triggered tremendous efforts to design other ZBG (Jacobsen
et al., 2007, 2010). Of note, the zing binding group affinity is as follows: hydroxamate >
retrohydroxamate > sulfhydryl > phosphinate > carboxylate > heterocyclic core.
Nevertheless, a zinc binding group with lower affinity may be advantageous.
In this line, we have proposed various hydrazide and sulfonylhydrazide-type functions as
potential ZBG. The sulfonylhydrazide derivative is responsible for the increased acidity of
the NH close to SO2 function allowing the H-bond to be formed with the catalytic glutamate
residue (Augé et al., 2003, 2004).
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
69
The hydroxamate acts as a bidentate ligand with the zinc ion to form the distorded trigonalbipyramidal coordination geometry. With respect to the design of new ZBG, a DFT (Density
Functional Theory) study revealed different modes of chelation of the sulfonylhydrazide
group (Rouffet et al., 2009).
The zinc ion was found to be ligated to three 4-Me-imidazoles used as mimetics of histidine
imidazole moieties located in the MMP catalytic site in physiological conditions. The
sulfonylhydrazide group could chelate the zinc ion in two different manners either in a bior tri-dentate mode.
In all cases, interaction between
i.
ii.
the Zn2+ ion and the sulfonamide nitrogen was observed as well as,
the Zn2+ ion with the oxygen atom of the sulfonylhydrazide carbonyl.
In the case of the tridentate mode, the third interaction involves one of the sulfonyl oxygen
atom and the Zn2+ ion. Consequently, the bidentate conformation was more favourable (4 to
5 kcal/mol) and the sulfonylhydrazide function seemed to possess ideal zinc binding
properties and also biodisponibility and stability.
Following these investigations, the sulfonylhydrazide group was incorporated into the
galardin® backbone as zinc binding group. Among the synthesized subtituents, the pbromobiphenyl group displayed a good potency for MMP-2 (LeDour et al., 2008). Then,
based on our preliminary results further modifications of the P’1 (long alkyl chain) and the
P’2 (phenyl group) substituents were introduced to increase the selectivity for MMP-2
(Figure 6).
Zn2+
Br
O
N
N
S
H
O
O
5
R1 = H, Ph
H
N
O
N
H
O
R1
Me
N
H
Fig. 6. Modifications of the P’2 group and sulfonylhydrazide function as ZBG.
Finally, a high potency for MMP-9 was obtained with the compound 5a (or 5b) with a small
group such as an isobutyl (Table 5). Introduction of a phenyl group at the position 2 of the
indole ring did not modify the activity. This could easily be explained taking into account
the S’2 solvent-exposed pocket.
Compounds
R1
MMP-1
MMP-2
MMP-3
MMP-9
MMP-14
5a
H
30
98
5800
3
20000
5b
Ph
350
247
53
18
1237
Table 5. Influence of the P’2 group and ZBG on the selectivity and potency of MMP
inhibitors. IC50 values are expressed in nM.
70
Enzyme Inhibition and Bioapplications
Docking studies of 5a confirmed the occupancy of the MMP-9 S’1 pocket by the isobutyl
group, the S2 subsite by the p-bromobiphenyl group and the chelation of the
sulfonylhydrazide to the catalytic site (Figure 7). It is known that the amino acid residues
Glu412 and Asp410, located in the S2 subsite control the selectivity of MMP-2 and MMP-9,
respectively.
In MMP-2, the amino acid residue Glu412 is able to form an H-bond with the substrate which
could not be formed in MMP-9 presenting the amino acid residue Asp410 (Chen et al., 2003).
In our case, no H-bond can be formed with the hydrophobic p-bromobiphenyl group and
the compound 5a displayed selectivity for MMP-9.
Fig. 7. Complex between MMP-9 and analogue 5a. The MMP-9 secondary structure was
represented in cartoon. The analogue 5a is shown with sticks in which the C atoms are
colored in magenta. Zn atom is displayed as a grey sphere.
Unfortunately, none of the P’1 (with an unsaturation and long alkyl chain or bulky
substituent) and P’2 (with a phenyl group) modified galardin® derivatives exhibited
increased inhibitory capacity and selectivity. Our docking data indicated that these
compounds adopted a conformation in which sp2-hybridized carbon atom of the alkylidene
side-chain led to steric hindrance impeding the entrance in the S’1 subsite. Consequently,
when the S2 pocket is occupied, the primed subsites could not tolerate any large substituent
and no synergistic effect could be obtained.
5. Control of elastolytic cascade by oleoyl-galardin®
Elastin degradation is at the genesis of cardiovascular disease as athero-arteriosclerosis and
aneurysm formation, and pulmonary diseases as chronic obstructive pulmonary disease or
lung cancer (Moroy et al., 2012; Muroski et al., 2008; Thompson & Parks, 1996).
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
71
Elastolysis requires the participation of serine- and metallo-elastases (Figure 8) which act
through proteolytic cascades.
d-hP I
d-hPI
 1P i
H LE
d-hPI
T IM P - 1:pro-M M P -9
M M P- 9
pr o-M M P -9
d-hP I
d-hPI
d-hPI
d-hP I
M M P- 3
M M P- 2
d-hP I
plasm in
pr o-M M P -3
d-hP I
pr o-M M P -2
Fig. 8. Control of the serine (HLE)- and metallo-elastases (MMP-2, MMP-9) crosstalk in
elastolysis by double-headed protease-MMP inhibitor (d-hPI).
Besides, a serine elastase as human leucocyte elastase (HLE) can degrade TIMP, and
reversely MMP can hydrolyse serine protease inhibitor as 1 proteinase inhibitor (Nunes et
al., 2011).
To control elastolysis, we thus attempted to design substances that could interfere with all
actors of the depicted cascade. For that purpose, long chain-unsaturated fatty acids, as oleic
acid, have been described to inhibit HLE (Hornebeck et al., 1985; Shock et al., 1990; Tyagi &
Simon, 1990) and to impede plasmin-mediated prostromelysin-1 activation (Huet et al.,
2004) as well as gelatinases activities (Berton et al., 2001). To that respect, we envisaged the
synthesis of a double-headed protease-MMP inhibitor (d-hPI) able to block elastase and
MMP activities. To that end, an oleoyl group was incorporated to galardin® at the P’3
position (Figure 9) (Moroy et al., 2011).
O
H
N
R
O
N
H
O
R = OH, 6
R = NH-OH, 7
MMP
NH
O
N
H
HLE, plasmin
Fig. 9. Double-headed protease-MMP inhibitor (d-hPI).
72
Enzyme Inhibition and Bioapplications
Oleoyl analogues (carboxylic 6 or hydroxamic 7 acids) are more potent than oleic acid to
inhibit MMP (Table 6). The hydroxamic acid 7 was found to improve the inhibitory capacity
toward MMP-2 comparing to oleic acid.
Compounds
MMP-2
MMP-3
MMP-7
MMP-9
HLE
Plasmin
Oleic acid
4.3
-
6.5
6.4
3.0
3.5
6
0.5
0.5
0.07
0.7
0.6
0.7
7
0.1
0.05
0.6
0.04
8.7
8.3
Table 6. Inhibition of MMP and serine elastases by oleic acid and oleoyl-galardin®
derivatives. MMP (Kis values are expressed in M), HLE and plasmin (IC50 values are
expressed in M).
The molecular docking computations indicated that compound 7 is able to bind MMP-2
active site and unable to chelate Zn2+ ion in the active site (Figure 10).
Fig. 10. Complex between MMP-2 and analogue 7. The MMP-2 secondary structure was
represented in cartoon. The analogue 7 is shown with sticks in which the C atoms are
colored in magenta. Zn atom is displayed as a grey sphere.
Instead, the hydroxamic acid function forms a salt bridge with the N-terminal end of the
amino acid residue Tyr110, while the long alkyl chain was inserted into the S’1 pocket as
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
73
was already demonstrated. The heterocycle is inserted into the S1 subsite where it interacts
via an H-bond with the carbonyl group of the amino acid residue Ala194 peptide bond.
We analyzed the inhibitory capacity of oleic acid, analogues 6 and 7 towards HLE and
plasmin activities. The compound 6 displayed high potency (IC50 = 0.6 M) against HLE, but
lower inhibition was observed with oleic acid and the analogue 7 (IC50 = 3.0 and 8.7 M,
respectively). Molecular docking computations indicated that the carboxylic function of
compound 6 and oleic acid can form a salt bridge with the amino acid residue Arg217, but
not compound 7 (not shown).
Almost the same values are found in the same order for the plasmin–mediated pro-MMP-3
activation. The lowest energy model of oleic acid with the kringle 5 domain is characterized
by the presence of a salt bridge between the carboxylic function and the amino acid residue
Arg512 of plasmin (not shown).
6. Control of gelatinases through impeding enzyme-substrate interaction
Another approach to control the activity of those enzymes consists in the regulation of
exosite protein-ligand interaction. To that respect both Fn(II) [or CBD] and PEX domains are
involved and, in keeping with data presented on Figure 2, blocking either Fn(II) or PEX
function will prevent the catalytic function of gelatinases on protein substrates selectively.
For instance, the proteolysis of collagen and elastin might be inhibited while maintaining
intact the potential of gelatinases to cleave proteoglycans or several growth factors.
6.1 Fn(II) domains
Using recombinant Fn(II) domain as bait, a one bead one-peptide combinatorial peptide
library was screened (Xu et al., 2007). A peptide displaying high sequence identity with the
segment 715-721 in human 1(I) collagen chain was identified and proved to inhibit by >
90% gelatinolysis catalyzed by MMP-2 (Xu et al., 2009).
The unsaturated fatty acid such as oleic acid inhibited MMP-2 with Ki = 4.3 M. Molecular
modelling studies focus on the interactions localized at two sites on MMP-2: the fatty chain
filled the S’1 pocket while the carboxylic acid group was exposed to the solvent. This result
is in agreement with our previous works showing that the S’1 pocket could accommodate
long alkyl side chains (LeDour et al., 2008; Moroy et al., 2007). The molecular docking
computations identified the second site of the oleic acid interactions as the 3rd Fn(II) domain.
The carboxylic acid function interacts via an H-bond with the phenolic group from the
amino acid residue Tyr381, via a salt bridge with the guanidinium group of the amino acid
residue Arg385 and via van der Waals interactions with the amino acid residue Leu356
while the unsaturated bond forms van der Waals interaction with the amino acid residues
Phe355, Trp374, Tyr381 and Trp387 (Figure 11).
Another approach could be the use of a more specific inhibitor directed against the 3rd Fn(II)
domain.
For this latter, an inhibitor with two carboxyl groups at each end of the alkyl chain should
be efficient for targeting respectively the amino acid residue Arg385, as it was observed for
74
Enzyme Inhibition and Bioapplications
oleic acid, and the amino acid residue Arg368 that is also present on the rim of the
hydrophobic pocket.
Interestingly, in the full MMP-2, the amino acid residue Arg368 forms a salt bridge with the
amino acid residue Asp40 belonging to the propeptide domain. According to the binding
mode of oleic acid, the size of the alkyl chain should be composed by 15 or 16 atoms of
carbon such as (7Z)-hexadec-7-enedioic acid and (6Z)-pentadec-6-enedioic acid.
Fig. 11. Complex between MMP-2 and oleic acid. Oleic acid is anchored in the binding
pocket at the surface of the 3rd Fn(II) domain.
6.2 Dual occupancy of the enzyme active site and Fn(II) domains
Another more complex strategy relies on the design of a dual occupancy of the enzyme
active site and the exosite as Fn(II). That has been originally attempted with a coupled
hydroxamate-based inhibitor to gelatin-like structures (Jani et al., 2005). No increase in
selectivity or potency of those compounds towards gelatinases could be attained; it was
attributed to the possibility that the Fn(II) and catalytic domains of enzyme are tumbling
independently.
Thus, we have built a hypothetical inhibitor from the inhibitors previously studied. We have
added alkyl groups until the hydrophobic pocket of the 3rd Fn(II) was reached: 19 are
needed to interact with its rims, i.e. the amino acid residues Arg368 and Trp387. If we want
to reproduce the binding mode of the oleic acid, 32 alkyl groups should be added (Figure
12). However, the size and the high flexibility of this kind of inhibitor could be problematic.
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
75
Fig. 12. Hypothetical model of an inhibitor able to bind both the catalytic site and the Fn(II)
domain of the human MMP-2. The MMP-2 is displayed with its accessible surface area. The
active site and the binding pocket on the 3rd Fn(II) domain are coloured in orange and in
cyan, respectively.
6.3 PEX domains
Since gelatinases are critically involved in directing cellular invasion, interfering with PEXintegrins (receptors) interaction might be a nice alternative. As an example, the use of phage
display identified a peptide that inhibits the association of MMP-9 PEX domain with the
v5 integrin, preventing proenzyme activation and cell migration (Björklund et al., 2004).
More recently, a 20 mers peptide encompassing the PEX-binding tail region of C-TIMP-2
was found to inhibit the membrane-mediated activation in HT-1080 cells (Xu et al., 2011).
PEX domains play a crucial function in the non proteolytic function of gelatinases. As
example, the PEX domain of MMP-9 is directly involved in the modulation of epithelial cell
migration in a transwell chamber assay (Dufour et al., 2008). This domain also promotes B
cell survival by interacting with 41 and CD-44 receptors (Redondo-Muñoz et al., 2010).
7. Conclusion
General considerations need to be pinpointed at aims to give a novel expansion to the
design of substances able to regulate MMP activity.
First, MMP as gelatinases are produced by nearly all cell types, but their cellular source may
intervene in their function and activity. Proteolytic activity liberated by activated
neutrophils is one pivotal element in the genesis and progression of aneurysms or chronic
obstructive pulmonary diseases (Muroski et al., 2008; Thompson & Parks, 1996).
76
Enzyme Inhibition and Bioapplications
It has been demonstrated that pro-MMP-9 is produced by neutrophils as a free form i.e. not
associated with TIMP-1 molecule, more readily activatable by enzyme as stromelysin-1
(Ardi et al., 2007). In addition, following activation, those cells release extracellular traps i.e.
neutrophil extracellular traps (NET) formed by the association of chromatin and granule
proteins; NET are enriched in neutral endopeptidases as neutrophil elastase and MMP-9
(Brinkmann et al., 2004). To that respect, the use of double-headed (HLE-MMP-9) inhibitors
as oleoyl-galardin® might be of therapeutic value. Advantageously, as we recently
documented, oleoyl moiety might be replaced by -lactam (Moroy et al., 2012), a more
potent and selective HLE inhibitor.
Although inflammation can orchestrate cancer (Kessenbrock et al., 2010), MMP-2 and MMP9 intervene in several other stages of cancer progression. Both enzymes have been involved
in promoting cell growth; MMP-2 is more linked to cancer cell invasiveness while MMP-9
may contribute to cell survival.
Up to now, one selective MMP-9 inhibitor is a monoclonal antibody binding to N-terminal
part of catalytic domain (Martens et al., 2007). Thus, intuitively, in keeping with those
distinct functions, the concept of selective inhibitor among gelatinases is emerging.
Their role in establishing a “metastatic niche” has also been delineated and their
contribution in angiogenesis has been widely underlined.
However, paradoxically, MMP-9 can generate either pro- or anti-angiogenic signals
(Figure 13).
On one side, it can
i. regulate VEGF bioavailability for VEGF-R2 receptor (Bergers et al., 2000),
ii. activate the basic fibroblast growth factor-2 (FGF-2) pathway (Ardi et al., 2009),
iii. generate elastin fragments i.e. elastokines with potent angiogenic activity (Robinet et al.,
2005).
PRO-ANGIOGENIC SIGNALS
ANTI-ANGIOGENIC SIGNALS
VEGF
pathway activation
Hinge
CBD
Elastokines
Elastin
FGF-2
pathway activation
catalytic
PEX-like
FnFn Fn Zn2+
His His
His
MMP-9
OG
Plasminogen
ANGIOSTATIN
Collagen IV3
TUMSTATIN
S
S
OGOGOG
Fig. 13. The paradoxical function of MMP-9 in angiogenesis.
At the opposite, proteolysis of plasminogen and 3 chain of collagen IV leads to the
formation of angiostatin and tumstatin (Cornelius et al., 1998; Kessenbrock et al., 2010).
Importantly, mice deficient in MMP-9 evidenced an increased-tumor growth which was
attributed to lack of tumstatin formation (Hamano et al., 2003).
As mentioned, both gelatinases exerted their action at the pericellular environment,
following binding of their PEX domain to receptors as v3 for MMP-2 or CD-44 for MMP-9.
Pharmacomodulation of Broad Spectrum Matrix
Metalloproteinase Inhibitors Towards Regulation of Gelatinases
77
Peptide or chemical libraries can be developed at aims to impede MMP-2(PEX)-v3 or
MMP-9(PEX)-CD-44 interactions.
As one example, a bivalent derivatized dilysine tetraamide was isolated which proved to
interfere with MMP-2/v3 interaction and inhibit angiogenesis (Silletti et al., 2001).
Possibly, this compound could be chemically modified to confer it additionally MMP-2
inhibitory activity (Bourguet et al., 2009).
One main problem related with the control of MMP in general and gelatinases in particular
relies on the kinetics of production of those enzymes during the cancer course from
initiation to metastasis formation. In other words, at what stage for one particular type of
tumor do we need to incorporate MMPI in cancer treatment?
The development of imaging MMP activity using derivatized selective inhibitor will
probably answer to this question. Several techniques have been already developed using
Positron Emission Tomography (PET) with 18F-labelled MMP-2 inhibitor (Furumoto et al.,
2003), Single Photon Emission Computed Tomography (SPECT) with a 123I gelatinases
inhibitor (Schaffers et al., 2004), or the use of fluorogenic substrates bearing self quenched
and near infrared FRET pairs (Scherer et al., 2008).
In our Federal Research Institute (IFR), we aim to develop hybrid nanoprobes build from
MMPI and fluorescent nanocrystal quantum dots (QDs). Design and chemical synthesis of
derivatives of galardin®, selective inhibitors of MMP-2, will be followed by their tagging
with QDs. Photo- and chemical stability of QDs will enable long-term spatiotemporal
tracking of the process of inhibition of MMP-2 with developed nanoprobe thus permitting
understanding of physiological process of invasion of melanoma for example.
8. Acknowledgment
Authors thank Université de Reims Champagne-Ardenne, IFR53 “Biomolécules”, Région
Champagne-Ardenne, EU (Fonds Feder) and CNRS for financial support. Technical assistance
of Mrs. M. Decarme is greatfully acknowledged.
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4
Non-Enzymatic Glycation
of Aminotransferases and the
Possibilities of Its Modulation
Iva Boušová, Lenka Srbová and Jaroslav Dršata
Department of Biochemical Sciences, Charles University in Prague,
Faculty of Pharmacy in Hradec Králové
Czech Republic
1. Introduction
Enzymes are catalytic protein molecules performing specific functions in their native form.
Native structure is one of basic conditions for normal function of proteins including
enzymes. Catalytic activity of enzyme may be decreased by both non-covalent modulation
by true inhibitors and covalent modifications by metabolites in the body or natural
products. One example of such modulation is non-enzymatic glycation of proteins by
sugars.
Enzymes are very good models for studies of protein interactions with other molecules,
including sugars (e.g., Arai et al., 1987; Dolhofer & Wieland, 1978; Okada et al., 1994; Okada
et al., 1997; Sakurai et al., 1987). Advantage of these studies is the fact that enzyme
interactions may be investigated not only by common methods of studies of protein
properties like changes in spectral characteristics, molecular weight, charge, solubility etc.
but also by measurement of their catalytic activity as the most characteristic property of
enzyme molecules. For such studies, it is important to use the enzyme with known structure
and reaction mechanism, available in sufficient purity, and measurable by simple assay
method. Aminotransferases belong to such enzymes. That is the reason why research
activities of our laboratory devote to inhibition of aminotransferases by low-molecular
compounds for many years (e.g. Dršata & Veselá, 1984; Netopilová et al., 1991; Netopilová
et al., 2001). Our studies of aminotransferase glycation belong to the efforts made also by
several other research groups (see e.g. Okada & Ayabe, 1997; Fitzgerald et al., 2000; Seidler
& Kowalewski 2003; Hobart et al., 2004).
1.1 Aminotransferases
Aspartate aminotransferase (AST, EC 2.6.1.1) and alanine aminotransferase (ALT, EC
2.6.1.2), enzymes frequently assessed in clinical laboratories, catalyse reversible transfer of
α-amino group from amino acids aspartate and alanine to the acceptor 2-oxoglutarate,
respectively. The resulting products are oxaloacetate, pyruvate and glutamate. In
metabolism, AST activity mediates the connection between the metabolism of amino acids
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Enzyme Inhibition and Bioapplications
and saccharides and participates in the transportation of reduced equivalents across the
membrane of mitochondria as a part of the malate–aspartate shuttle. The catalytic activities
of AST in cells are implemented by two isoenzymes—the cytosolic and the mitochondrial
one, which differ in primary structure and in some properties (Yagi et al., 1985; Metzler et
al., 1979).
Porcine heart cytosolic AST, which was used in further described experiments of our
research group, is a homodimer with molecular mass of about 93,150 Da. Both subunits are
composed of 412 amino acid residues. This dimer contains 38 lysine and 52 arginine
residues with six Lys–Arg and four Arg–Lys sequence pairs (Seidler & Kowalewski, 2003).
Presence of these amino acid residues makes glycation of aminotransferases in vitro as well
as in vivo possible. The Lys258 binding coenzyme PLP could be one of the possible targets of
glycating agents as can be seen from the loss of enzymatic activity due to the effect of
methylglyoxal as well as other glycating agents (e.g. Boušová et al., 2009; Seidler &
Kowalewski, 2003).
Aminotransferases are characterised by the presence of coenzyme pyridoxal-5’-phosphate
(PLP) and its direct participation in catalysis. The protein part of aminotransferase
molecules consists of two identical, non-covalently bound subunits, which are composed of
one small and one large domain. PLP coenzyme forms a Schiff base with ε-amino group of
lysine residue (Lys313 in ALT and Lys258 in AST) located in an active site of the larger
domain of each subunit (Kirsch et al., 1984). The PLP form of AST shows, depending on pH,
a major absorption peak at 360 nm (an active, unprotonized form of the coenzyme,
prevailing at lower pH values), and/or a peak at 430 nm (an inactive, protonized form,
increasing at lower pH values) (Kirsch et al., 1984). After a reaction with L-aspartate during
the first part of a ping-pong transaminating reaction, the pyridoxamine-5’-phosphate (PMP)
form of the coenzyme appears and the original absorption maximum shifts to 325–330 nm
(Fig. 1A). While free PLP or PMP are not optically active substances, the coenzyme bound in
the active site of AST shows circular dichroism (CD) spectra in the range of 300–500 nm,
which are similar to absorption spectra (Fig. 1C). The CD effect is caused by the change in
the electronic configuration of the molecule (Kelly & Price, 2000). Circular dichroism clears
away absorption characteristics of optically inactive components, which facilitates
identification of the specific coenzyme signal and its changes (Dršata et al., 2005). This
method is also a powerful tool for characterizing secondary (190-240 nm, chromophore is
peptide bond) and tertiary (250-320 nm, chromophores are aromatic amino acids and
disulfide bonds) structures of studied proteins as well as determining whether the protein is
folded (Kelly & Price, 2000).
1.2 Non-enzymatic glycation of proteins
Non-enzymatic glycation, also called Maillard reaction, was first described in 1912 by Louis
Camille Maillard (Monnier, 1989). This non-enzymatic browning process had been first
extensively studied by food chemists and later has become the center of attention of
geological or agricultural sciences. Much later it was recognized that the process is
important for medical science (Monnier, 1989; Singh et al., 2001). A very common but
serious human disease is diabetes mellitus, which in its untreated or unsuccessfully treated
form is accompanied with development of chronic complications due to increased intra- and
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
87
Fig. 1. Characteristic UV-VIS absorption and CD spectra of AST (panel A and C) and of the
free coenzyme (panel B). (A) UV-VIS spectra of AST (1 mg/mL; 0.1 M sodium phosphate
buffer, pH 7.4) alone or in the presence of L-Asp (1 mM), (B) UV-VIS spectra of free PLP (0.1
mM) and PMP (0.1 mM) in sodium phosphate buffer (0.1 M; pH 7.4) were recorded using a
diode array spectrophotometer HP 8453 (Hewlett Packard, USA) in a 0.5 cm quartz cuvette
against sodium phosphate buffer (0.1 M; pH 7.4). (C) CD spectra of AST (1 mg/mL; 0.1 M
sodium phosphate buffer, pH 7.4) alone or in the presence of L-Asp (1 mM) were recorded
using a dichrograph CD6 (Jobin Yvon, France) in a 0.5 cm quartz cuvette against sodium
phosphate buffer (0.1 M; pH 7.4). For further details on experimental work see
(Dršata et al., 2005)
extracellular concentration of glucose including a hallmark of the disease - hyperglycemia.
There is increasing evidence that Maillard reaction plays an important role in the onset and
progression of some other diseases, such as atherosclerosis and Alzheimer’s disease
(Nursten, 2005; Yegin et al., 1995). Plenty of studies have been devoted to investigation of
protein structural and functional changes caused by glycation process (e.g. Jabeen &
Saleemuddin, 2006; Seidler & Seibel, 2000; Yan & Harding 1997, 2006; Zeng et al., 2006; Zhao
et al., 2000).
1.2.1 Formation of advanced glycation end-products
The early stage of the Maillard reaction is initiated by non-enzymatic condensation of a
reducing sugar or a certain related compound (e.g. ascorbate) with a free amino group of a
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Enzyme Inhibition and Bioapplications
protein, a lipid or a nucleic acid. In the case of glucose, the reaction first leads to the formation
of acid-labile Schiff base, which undergoes a rearrangement to a relatively stable Amadori
product, e.g. fructosamine. Only a small portion of these Amadori-adducts experiences further
rearrangements leading to an irreversible formation of advanced glycation end-products
(AGEs) (Monnier, 1989). The reaction with fructose proceeds in a similar way, but it is called
Heyns rearrangement and two separate Heyns products are generated (Suarez et al., 1989).
The formation of Schiff base proceeds in the range of hours and it is fully reversible, while
Amadori rearrangement takes days and is reversible only to a certain extent.
In the intermediate stage, Amadori product subsequently degrades and various reactive
intermediates are formed. These products are known as α-dicarbonyls or α-oxoaldehydes
and are represented by products like methylglyoxal (MGO), 3-deoxyglucosone (3-DG), and
glyoxal (GO). Also a Schiff base is a potential source of reactive α-dicarbonyls, because it can
be fragmented to MGO and GO. These dicarbonyls possess higher reactivity towards
proteins than the parent monosaccharide. They are capable of forming various cross-links as
well as chromo/fluorophoric adducts called AGEs, upon reaction with proteins (Schalkwijk
et al., 2004; Wolff et al., 1991). Both MGO and 3-DG form adducts with proteins and nucleic
acids up to 10,000 times more readily than glucose (Beisswenger et al., 2003). The
accumulation of α-dicarbonyl compounds is termed carbonyl stress (Miyata et al., 1999). The
other process proceeding during the intermediate stage of glycation is metal catalyzed
autoxidation of glucose, in which the carbonyl compounds (arabinose and glyoxal), H2O2
and free radicals are formed (Hunt et al., 1988; Wolff & Dean, 1987). The generated free
radicals initiate further oxidative steps. The glycation process accompanied by oxidation
steps is called glycoxidation (Baynes, 1991). Various pathways incorporated in the formation
of AGEs are shown in Fig. 2.
The advanced glycation end-products are formed during the late stage of glycation over a
period of weeks, thereby affecting predominantly long-lived proteins, such as collagen and
lens crystallins. They represent a heterogeneous group of compounds rising from different
precursors. The chemical structures of AGEs have not been fully described yet. These
compounds are formed either by oxidative pathway (pentosidine and CML) or by nonoxidative pathway (pyrraline, DOLD, GOLD, MOLD, and CEL) as can be seen in Fig. 2.
Proteins modified by advanced glycation are characterized by a much higher molecular
weight than the original protein, a yellow-brown pigmentation, a typical fluorescent spectra
(λex/λem: 370/440 nm), an ability to form various cross-links, and by their biological half-life,
which is comparable to the half-lives of parent proteins (Lapolla et al., 2005; Singh et al., 2001).
Glucose is the least reactive of the common sugars and that is probably the reason for its
evolutionary selection as the principal sugar in vivo (Bunn et al., 1978). Because of its low
reactivity towards proteins, AGEs have been thought to form only at long-lived extracellular
proteins, such as collagen, crystallines, and myelin. Recently also rapid intracellular AGE
formation by various intracellular sugars (e.g. fructose, ribose, glyceraldehyde,
dihydroxyacetone phosphate, glyceraldehyde-3-phosphate, glyoxal, methylglyoxal, and 3deoxyglucosone) in vivo has been described. The rate of glycation is directly proportional to
the percentage of sugar in the open-chain form and the rate for fructose (0.7% open-chain) is
7.5-fold faster than that of glucose (0.002% open-chain). More strikingly, the glycolytic
intermediate glyceraldehyde-3-phosphate (100% open-chain) forms 200-fold more glycated
proteins than do equimolar amounts of glucose (Schalkwijk et al., 2004).
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
89
Fig. 2. Three stages of non-enzymatic glycation reaction. CML, N-ε-(carboxymethyl)lysine;
GOLD, glyoxal-lysine dimer; CEL, N-ε-(carboxyethyl)lysine; MOLD, methylglyoxal-lysine
dimer; 3-DG, 3-deoxyglucosone; DOLD, deoxyglucosone-lysine dimer
1.2.2 Structure of AGEs
The advanced glycation end-products found under physiological conditions can be
classified according to their fluorescent properties and their ability to form cross-links. The
first group is represented by fluorescent AGE cross-links, which are thought to be
responsible for a major share of the deleterious effects of AGEs in diabetes and aging.
Fluorescence is a good qualitative indicator used to estimate AGEs formation. Pentosidine,
crossline, and various vesperlysines are members of this group. However, also nonfluorescent AGE cross-links are found in vivo. Their isolation and identification is more
complicated than in the case of fluorescent AGE cross-links. It is thought that they account
just for 1% of all cross-links rising under physiological conditions. Various imidazolium
dilysine cross-links (GOLD, MOLD), arginine-lysine cross-links, and glucosepan belong to
this group. Last but not least, a group of non-cross-linking protein bound AGE structures
have been identified in vivo. These structures may exert deleterious effects as precursors of
cross-links or as biological receptor ligands inducing a variety of adverse cellular and tissue
changes. The well-known members of this group are pyrraline, carboxyalkyllysines (CML,
CEL), imidazolones, and argpyrimidine (Ulrich & Cerami, 2001). Classification and
examples of each above mentioned group are shown in Fig. 3.
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Enzyme Inhibition and Bioapplications
Fig. 3. Classification of AGEs formed under physiological conditions including several
examples to each group. [Lys] represents a desamino-lysine residue; [Arg] stands for a
desguanidino-arginine residue; R represents either hydrogen atom (GOLD), methyl group
(MOLD), 1,2,3-trihydroxypropyl (DOLD) or 2,3,4-trihydroxybutyl group (imidazolone A and B).
1.2.3 Therapeutic strategies targeting the AGEs
The therapeutic intervention to the glycation process has followed three main approaches. A
first approach follows inhibition of RAGE by neutralizing antibodies or suppression of postreceptor signaling using antioxidants. A second one is inhibition of AGE formation process
by carbonyl-blocking agents (aminoguanidine) or by antioxidants. The last approach is
reducing AGE deposition by using cross-link breakers or by enhancing cellular uptake and
degradation.
Interactions of AGEs with the receptor for AGEs (RAGE) have been implicated in the
development of diabetic vascular complications, which cause various disabilities and
shortened life expectancy, and reduced quality of life in patients with diabetes. These
undesirable effects can be suppressed by the use of specific antibodies to RAGE, soluble
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
91
RAGE or by suppression of post-receptor signaling using antioxidants (Hudson et al., 2003;
Stuchbury & Münch, 2005). The secreted RAGE form, named soluble RAGE (sRAGE), acts
as a decoy to trap ligands and prevent interaction with cell surface receptors (Bucciarelli et
al., 2002). Soluble RAGE was shown to have important inhibitory effects in several cell
culture and transgenic mouse models, in which it prevented or reversed full-length RAGE
signaling. The administration of sRAGE has been shown to suppress accelerated diabetic
atherosclerosis (Park et al., 1998).
Aminoguanidine, also known by its trade name Pimagedine (Alteon Inc.), is a prototype
therapeutic agent for prevention of the AGEs formation. It is a low-molecular, highly
nucleophilic hydrazine compound that rapidly reacts with α-dicarbonyl compounds such as
MGO, GO, and 3-DG to prevent formation of AGE cross-links. The products of the
scavenging reaction are substituted 3-amino-1,2,4-triazines. Aminoguanidine does not affect
the formation of the Schiff base and Amadori products (Thornalley et al., 2000). Clinical
trials of aminoguanidine in overt diabetic nephropathy (ACTION) were performed, but they
were early terminated due to safety concerns. Reported side effects of aminoguanidine in
clinical therapy were gastrointestinal disturbance, abnormalities in liver function tests, flulike symptoms, and a rare vasculitis (Bolton et al., 2004; Thornalley, 2003). Other
nucleophilic compounds, which are designed to trap reactive carbonyl intermediates in
AGE formation, are for example OPB-9195, diaminophenazine, tenilsetam, and
pyridoxamine (Baynes & Thorpe, 2000; De La Cruz et al., 2004). With regard to the presence
of free radicals and oxidative steps in the course of glycoxidation, compounds with
antioxidant effect such as α-lipoic acid, α-tocopherol, ascorbic acid, and ß-carotene were
tested. Dipeptide carnosine, pyridoindole derivative stobadine, hypolipidemic drug
probucol, and mucolytic remedy N-acetylcysteine are just a few more examples of the
compounds with described antioxidant properties, which were tested in order to estimate
their potential protective effect in the process of glycation. Also some antioxidant enzymes
such as superoxide dismutase, catalase, and selenium-dependent glutathione peroxidase
may protect proteins against impairment caused by non-enzymatic glycation (De La Cruz et
al., 2004; Kyselova et al., 2004).
Aminoguanidine and other compounds mentioned before can inhibit the formation of new
AGE cross-links, but they are not able to cleave those already formed. Vasan et al. (1996)
reported the first of cross-link breakers, phenyl thiazolium bromide (PTB). This anti-AGE
agent chemically breaks α-dicarbonyl compounds by cleaving the carbon-carbon bond
between the carbonyls. Under physiological conditions, PTB is not stable and therefore its
analogs were tested and alagebrium chloride (ALT-711), a highly potent cross-link breaker
with higher stability, has been discovered. This compound successfully completed
preclinical studies and Phase II clinical study on healthy volunteers (Yamagishi et al., 2008).
Unfortunately, the specific types of AGEs affected by alagebrium are more important in rats
than humans; hence the promising results in animals were never repeated in human studies.
Randomized Phase II clinical trial (BENEFICIAL) in patients with chronic heart failure
(Willemsen et al., 2010) has been terminated early due to financial constraints.
The objective of current study was to evaluate potential antiglycation activity of two
mitochondrial antioxidants, α-phenyl N-tert-butyl nitrone (PBN) and N-tert-butyl
hydroxylamine (NtBHA). PBN is a nitrone that traps free radicals and protects against
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Enzyme Inhibition and Bioapplications
damage in different models such as inflammation, ischemia reperfusion, and aging. Its
decomposition product NtBHA mimics PBN and is much more effective in delaying
senescence of human lung fibroblasts IMR90 (Atamna et al., 2000). NtBHA appears to act on
mitochondria to delay alterations in function (Atamna et al., 2001). Supplementation with
NtBHA improved the respiratory control ratio of mitochondria from liver of old rats
(Atamna et al., 2001).
1.3 Summary of existing results
Our laboratory deals with research of the protein glycation in vitro for many years.
Aminotransferases were chosen as suitable model proteins, because they are commercially
available in a highly purified and stable form, their structures and mechanism of catalysis
are well known, and at least two simple methods for their enzyme activity assessment are in
use. This subchapter summarizes results, which have been obtained by our research group
up to now.
In our laboratory, we found decrease in alanine aminotransferase (ALT, EC 2.6.1.2) activities
in the presence of several reducing monosaccharides in vitro. The decrease in the catalytic
activity of ALT from porcine heart after 20 days of incubation with D-glucose, D-fructose, Dribose or D,L-glyceraldehyde varied and was dependent on the nature of glycating agent
(the percentage of sugar present in open-chain form). The dependence of enzyme
inactivation on the presence of these sugars and time of incubation is presented in Fig. 4
(Beránek et al., 2001). As was described earlier, the rate of glycation is directly proportional
to the percentage of sugar in the open chain form and the rate for fructose (0.7% open-chain)
is 7.5-fold faster than that of glucose (0.002% open-chain). More strikingly, the glycolytic
intermediate glyceraldehyde 3-phosphate (100% open-chain) forms 200-fold more glycated
proteins than do equimolar amounts of glucose (Schalkwijk et al., 2004).
Fig. 4. An effect of glycation on ALT activity. ALT was incubated with 50 mM sugars in
sodium phosphate buffer (0.1 M, pH 7.4) at 25 °C. Aliquots of samples were taken at days 0,
2, 6, 9, 13, 20 and remaining enzyme activities in samples were determined. Activity was
expressed as a percentage of the activity of the control sample (without sugars). ● ALT with
D-glucose;  ALT with D-ribose; ALT with D-fructose; ■ ALT with D,L-glyceraldehyde
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
93
Fructose, ribose and glyceraldehyde have been found more potent glycating agents than
glucose. The strongest glycation effect was exerted by D,L-glyceraldehyde. Complete
enzyme inhibition was reached after 6 days but most enzymatic activity (about 75%) was
reduced in the course of the first 2 days of incubation. Moreover, a decrease in ALT activity
to 88% of the relevant control was apparent at the zero-time determination of the enzymatic
activity in the glyceraldehyde sample. Similar results were obtained with AST (Beránek et
al., 2002). These data are presented in Fig. 5.
Fig. 5. Inhibitory effect of glycation on the catalytic activity of aminotransferases incubated
in sodium phosphate buffer (50 mM, pH 7.4) at 4 °C (―――), 25 °C (––––) and 37 °C (–·–·–) for
up to 56 days. The values show residual catalytic activities of the enzymes related to the
appropriate control. (A) ALT incubated with 50 mM D-fructose; (B) AST with 50 mM Dfructose; (C) ALT with 500 mM D-fructose; (D) AST with 500 mM D-fructose.
In our further experiments, aspartate aminotransferase (AST, EC 2.6.1.1) has been chosen as
a model protein due to its relative stability during in vitro incubation and availability of the
enzyme preparations of high purity (for further experimental details see Dršata et al., 2005).
The purity of the enzyme preparation was crucial for experiments with prolonged
incubation. While AST activity of the rat liver 20 000 x g supernatant was very unstable in
vitro and declined rapidly independently of presence or absence of sugar during incubation
even at 25 °C, the purified preparation enabled experiments using incubation for many days
(Tupcová, 1996). Data are presented in Fig. 6.
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Enzyme Inhibition and Bioapplications
Fig. 6. Comparison of stability of AST in rat liver supernatant and in purified preparation
from pig heart during in vitro incubation at 25 °C. The incubation mixture was diluted
before the assay in order to obtain activities within the analytical range of the method.
Values are mean ± S.D. of three independent samples. Each sample was measured
three-times and the mean was used to calculate the value presented.
Moreover, glycation of AST was accompanied by a decrease in its catalytic activity in
dependence on the concentration and activity of the glycating agent, while the concentration
effect was not clearly demonstrated in case of ALT (Dršata et al., 2002). The effect of substrates
(2-oxoglutarate and L-aspartate) on AST stability and on glycation process was assessed as
well. There was no effect of 2-oxoglutarate on the control AST activity throughout the
experiment, which demonstrated a high stability of the pyridoxal form both in the presence
and absence of this substrate. On the other hand, the presence of 25 mM L-aspartate (inducing
the pyridoxamine form of the enzyme) caused a rapid decrease in AST activity even in the
control reaction (Dršata et al., 2002). The results of AST incubation with D-ribose and Dfructose in presence of 0.5 mM 2-oxoglutarate suggested that AST glycation could be partly
prevented by this substrate. This finding supports the idea that Lys258 in the active center of
AST is involved in glycation of the free enzyme (Fig. 7, taken from Dršata et al., 2002).
The in vitro model of protein glycation (AST) by D-fructose has been then established and
experimentally used. (Boušová et al., 2005a). As mentioned above, attempts have been
made by researchers to investigate various chemical compounds as potential antiglycating
agents. With this model, influence of potential antiglycating compounds with antioxidant
activities was investigated. In an attempt to reduce glycoxidation process and formation of
AGEs, influence of endogenous antioxidant uric acid (0.2-1.2 mM) on glycoxidation process
of AST by 50 mM and 500 mM D-fructose in vitro was studied. Uric acid at 1.2 mM
concentration reduced AST activity decrease caused by incubation of the enzyme with 50
mM sugar up to 25 days at 37 °C (Fig. 8), as well as formation of total fluorescent AGE
products (Fig. 9). The results obtained supported the hypothesis that uric acid has beneficial
effects in controlling protein glycoxidation (Fig. 8 and 9).
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
95
Fig. 7. Influence of substrates on AST activity during glycation by D-ribose in vitro.
Concentration of the enzyme (purified Serva preparation) in the incubation mixture: 0.413
mg/ml, final catalytic concentration in the mixture 7.83 µkat/mg. Values are mean ± S.D. of
6 (control) or 3 (with D-ribose) independent samples. Each sample was measured threetimes and the mean was used to calculate the value presented.
L-Asp = L-aspartate; 2-OG = 2-oxoglutarate
Fig. 8. Effect of glycation on AST activity and intervention of uric acid (UA) in the process.
AST (1.33 mg/ml) was incubated with and without fructose (Frc) in 0.1 mM phosphate
buffer, pH 7.4 at 37 °C in the presence or absence of 1.2 mM uric acid. AST activity was
expressed as percentage of that of control sample (without sugar), which was considered as
100% ± S.D. (%) at every interval. Each point represents an average of three experiments in
interval 0-12 days, and an average of two experiments on the 15th and 21st day. Each
experiment was performed in triplicate (*data with P<0.05, Student’s t-test).
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Enzyme Inhibition and Bioapplications
Fig. 9. Formation of fluorescent products of glycation under the conditions of long lasting
incubation. AST (1.33 mg/ml) was incubated with or without D-fructose in sodium
phosphate buffer (0.1 M, pH 7.4) at 37 °C in the presence or absence of 1.2 mM uric acid
up to 25 days. Aliquots of samples were taken on days 0, 1, 3, 5, 25, and arising
fluorescent AGE products in samples were determined at specific wavelengths of
excitation and emission (λex/λem) corresponding to total AGEs (370/440 nm). Data of
relative fluorescence were expressed in arbitrary units per mg of protein ± S.D., with 1 AU
corresponding to the fluorescence of BSA 1.0 mg/ml. Every point in days 0 and 5
represents an average of four experiments (10 samples), in days 1 and 3 an average of
three experiments for mixtures with 50 mM fructose (7 samples) and of two experiments
for mixtures with 500 mM fructose (4 samples), and in day 25 an average of three
experiments (7 samples), (*data with P<0.05, Student’s t-test).
Also further studies of our research group have been pointed at possible protective activity
of selected natural compounds (e.g. hydroxycinnamic acids, flavonoids, arbutin,
hydroxycitric acid) on AST glycation by fructose. The results have shown that these
compounds can exert also negative effects on enzyme activity but some of them have been
able to slow down the course of AST modification by glycating agent. The compound with
overall positive activity has been hydroxycitric acid (Fig. 10), which is major active
component in the fruit rinds of certain species of the plant Garcinia. The effect of this
compound was concentration-depended and its positive activity was most pronounced at
2.5 mM concentration. On the other hand, flavonoid baicalin (Fig. 11) and its aglycone
baicalein rapidly decreased the in vitro activity of the enzyme in all concentrations used (0.5–
3 mM), and no beneficial effects of these compounds on glycation of the enzyme by fructose
were found (Boušová et al., 2005b).
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
97
Fig. 10. The effect of hydroxycitric acid (HCA) on the glycation of AST by fructose in vitro.
Concentration of AST in incubation mixtures was 1.33 mg/ml. AST was incubated with or
without D-fructose 50 mM in sodium phosphate buffer (0.1 M, pH 7.4, 0.05% sodium azide)
at 37 °C in the presence or absence of HCA up to 21 days. The samples were diluted before
the assay to fit to the analytical range of the method. Values are expressed as a percentage of
the activity of respective control (AST) ± S.D. of six independent samples (*data with P<0.05,
Student’s t-test).  AST;  AST + Frc 50 mM; ▲ AST + Frc + HCA 2.5 mM; ∆ AST + Frc +
HCA 5.0 mM;  AST + Frc + HCA 7.5 mM; o AST + Frc + HCA 10.0 mM.
Fig. 11. Direct effect of baicalin on the AST activity in vitro. Concentration of AST in
incubation mixtures was 1.33 mg/ml in 0.1M sodium phosphate buffer (pH 7.4; 0.05%
sodium azide). Incubation at 37 °C. The samples were diluted before the assay to fit to the
analytical range of the method. Values are expressed as mean ± S.D. of six (control and AST
+ Frc 50 mM) or three (with baicalin) independent samples (*data with P<0.05, Student’s ttest).  AST;  AST + Frc 50 mM; ▲ AST + baicalin 0.5 mM; ∆ AST + baicalin 1.0 mM;  AST
+ baicalin 1.5 mM; o AST + baicalin 3.0 mM.
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Enzyme Inhibition and Bioapplications
Following experiments were conducted using methylglyoxal (MGO) as a glycating agent,
because this compound has higher glycating potential than reducing monosaccharides and
the incubation period has been shortened to one week only. Changes in the catalytic activity
of AST caused by MGO were observable even after 120 min of incubation at 37 °C.
Antiglycating activity of hydroxycitric (Fig. 12) and uric acid has been studied in this
modified model (Boušová et al., 2009).
Fig. 12. Effect of glycation on AST activity and its intervention by hydroxycitric acid. AST (5
µg/ml) was incubated with or without methylglyoxal (0.5 mM) in 0.1 M sodium phosphate
buffer (pH 7.4) at 37 °C in the presence or absence of hydroxycitric acid (1.0 and 2.5 mM).
Catalytic activity of AST was expressed as percentage of each sample activity at the time 0,
which was 100% ± S.D. (%). Every point represents an average of two independent
experiments, in which assays were performed in triplicates († data with P<0.05 and *data
with P<0.01, Student’s t-test).
2. Methods
Following parameters have been assessed: enzyme activity, fluorescence (total AGEs and
argpyrimidine), amount of primary amino groups, molecular charge of AST (using native
polyacrylamide gel electrophoresis), and protein cross-linking and aggregation (using
polyacrylamide gel electrophoresis under reducing conditions with subsequent western
blotting). Structures of all tested compounds are shown in Fig. 13.
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
99
Fig. 13. Structures of tested compounds
2.1 Sample preparation and incubation
The enzyme suspension was centrifuged at 5000 rpm at 4 °C for 20 minutes, the supernatant
was removed, and protein pellet was reconstituted in 0.1 M sodium phosphate buffer (pH
7.4, 0.05% sodium azide) and the stock solution of 1.0 mg/ml was prepared. This stock
solution was used for the preparation of four different types of incubation mixtures: (a)
control samples (with buffer only), (b) methylglyoxal-modified samples (with MGO in a
final concentration of 0.5 mM), (c) direct protein-antioxidant interaction samples (with
individual antioxidants in a final concentration 0.5-10 mM), (d) antiglycation samples (with
individual antioxidants in a final concentration of 0.5-10 mM and MGO in a final
concentration 0.5 mM). The inhibitory effect of α-phenyl N-tert-butyl nitrone and N-tertbutyl hydroxylamine on protein glycation was compared to the effect of aminoguanidine
(AG) in a concentration of 1.0 mM and Trolox in a concentration of 2.5 mM. The final
concentrations of the enzyme were 5 µg/ml for catalytic activity assessment and 0.5 mg/ml
for electrophoresis, amine content and fluorescence measurements. All incubation mixtures
were incubated in the dark at 37 °C for up to 14 days. The low-molecular compounds were
removed using Amicon centrifugal filtration device with 0.1 M sodium phosphate buffer
(pH 7.4), protein content was measured using Bradford assay, and adjusted to the
concentration 0.5 mg/ml. Aliquots were stored frozen at -20 °C until analysis. All samples
were assessed in triplicates and experiments were repeated twice if not stated otherwise.
2.2 Enzyme assay
Catalytic activity of AST was assessed spectrophotometrically using kinetic UV method
with addition of PLP (Bergmeyer et al., 1986). Sample aliquots were diluted by 0.1 M
sodium phosphate buffer (pH 7.4) to obtain enzyme activities within the analytical range of
the method used. Sampling and measuring was carried out at 37 °C in the intervals 0, 120,
and 240 minutes using Helios ß spectrophotometer. Absorbance changes at 340 nm were
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Enzyme Inhibition and Bioapplications
used to calculate enzyme activities. All results of enzyme assays were expressed in µkat/l
and usually recalculated as activities relative to those of the value of individual sample at
time 0.
2.3 Fluorescence measurements
Formation of fluorescent AGEs and argpyrimidine were measured using the method of Wu
& Yen (2005) with some modifications. Briefly, samples were incubated for 7 days at 37 °C.
The aliquots were taken away at time 0, 3 and 7 days and stored frozen at -20 °C. Aliquots of
time 0 were used as unincubated blanks. Fluorescence of samples was measured at
excitation and emission wavelengths of 330 nm/410 nm (fluorescent AGEs) and 320 nm/380
nm (argpyrimidine) against corresponding blanks in 96-well-plate by microplate reader
(Tecan Infinity M200) using 0.1 mg of protein per well. The percentage inhibition of AGEs
and argpyrimidine formation were calculated according to following formula: % inhibition
= [1 - (fluorescence of test group/fluorescence of glycated control)] x 100%.
2.4 Determination of primary amino groups
Amine content, which is a measure of protein glycation, was estimated spectrophotometrically
with trinitrobenzenesulfonic acid (Steinbrecher, 1987) using ß-alanine as the standard. Sample
containing 50 µg of AST was incubated with 0.1% trinitrobenzenesulfonic acid in alkaline
conditions for 2 h at 37 °C. The reaction was stopped by acidification (1 M HCl) and addition
of 10% sodium dodecyl sulfate. The absorbance of trinitrophenyl-amino acid complex was
measured at 340 nm. The standard curve was linear in the range 5–100 nmol of NH2.
2.5 Effect of glycation on molecular charge of AST
Native polyacrylamide gel electrophoresis (PAGE) was used to investigate the changes in
the molecular charge of AST due to glycation. Electrophoresis was performed in
discontinuous system with 4% stacking gel and 7.5% separating non-denaturating gel
(Ornstein, 1964; Davies, 1964). All lanes were loaded with 9 µg of protein. Electrophoresis
was performed at 30 mA for 2 hours using Mini ProteanIII apparatus. The gel was then
stained by colloidal Coomassie Blue G250, scanned, and relative migration distances were
calculated from Rf using Quantity One software. Electrophoretic mobilities were expressed
as a rise in percentage mobility compared to the native enzyme (control).
2.6 SDS-PAGE and western blotting
Protein cross-linking and aggregation were assessed using a sodium dodecyl sulfate
polyacrylamide gel electrophoresis (SDS–PAGE) on Mini ProteanIII apparatus (BioRad).
SDS-PAGE was performed using discontinuous system with 4% stacking gel and 10%
separating gel (Laemmli, 1970). Lanes were loaded with 4 µg of protein. Proteins after
electrophoretic separation were transferred to PVDF membrane (0.2 µm, Bio-Rad) at a
constant voltage 100 V for 90 minutes (Mini Trans-Blot Electrophoretic Transfer Cell, BioRad). After blotting, membranes were blocked with 8% non-fat dry milk in Tris buffered
saline-Tween-20 buffer (TBST) overnight at 4°C, then washed in TBST and reacted with
primary antibody (dilution 1:1000) for 45 minutes at room temperature. Subsequently,
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
101
membranes were washed six times with TBST and incubated with secondary antibody for
45 minutes (dilution 1:1000). The blots were extensively washed in 0.1 M TRIS buffer
containing 5 mM MgCl2.6H2O (pH 9.5), covered with chemiluminescent substrate DuoLux
(Vector Laboratories) and incubated for 5 minutes. The membranes were then exposed to Xray film (CL-XPosure film, Thermo Fisher Scientific), developed by standard developing
process, and images were recorded with a GelDoc XR system. The blots were
densitometrically quantified using Quantity One software.
2.7 Statistical analysis
Values of catalytic activity are given as means ± S.D. and mostly expressed in % of the time 0
of individual samples ± relative S.D. Values of fluorescence (AU) are given as means ± S.D.
Statistical significance was determined using Student’s t-test and differences were regarded
as significant when P<0.05 and P<0.01, respectively.
3. Results and discussion
The activity of tested compounds was compared to the effect of known carbonyl-blocking
agent aminoguanidine and to the effect of Trolox (water-soluble derivative of vitamin E),
which is often used in various methods for assessing antioxidant/antiradical properties of
potential antioxidants as reference compound.
3.1 Enzyme assay
Activity of control sample (AST alone) was stable throughout the experiment. Some of
tested compounds had a more or less pronounced negative direct effect on enzyme activity,
which was probably due to a direct interaction of their molecules with the molecule of the
enzyme. PBN itself had no harmful influence on stability and catalytic activity of AST in the
concentrations used (Fig. 14B), while NtBHA caused concentration-dependent decrease in
AST catalytic activity, which was statistically significant at concentrations of NtBHA 1 mM
and higher (Fig. 14A). Aminoguanidine 1.0 mM caused significant decrease of enzyme
activity by 21.7% after 240 min of incubation. This inhibitory effect of AG may be explained
by its binding to PLP coenzyme forming a Schiff base, which disturbs tissue distribution of
PLP in vivo and decreases its concentration in liver (Okada & Ayabe, 1995; Taguchi et al.,
1998). Trolox 2.5 mM did not influence AST activity.
Following incubation of enzyme with MGO 0.5 mM, a rapid decline of AST activity was
observed. The enzymatic activity decreased to 53.1 and 30.1% of control sample after 120
and 240 min, respectively. In addition, positive antiglycation effects were observed with
some compounds. NtBHA exerted antiglycation influence only at 5 mM concentration after
120 and 240 min and at 1 mM concentration after 240 min of incubation (Fig. 15A). Negative
direct effect of this compound observed at 10 mM concentration probably outweighed its
positive antiglycation activity. The catalytic activity of AST was by 33.1% and 16.5% higher
in the presence of PBN 10 mM and 1 mM after 120 min of incubation with MGO compared
to the activity of sample containing AST + MGO only (Fig. 15B), respectively. PBN 1-10 mM
protected AST against MGO-induced glycation also after 240 min of incubation, when all
three concentrations increased activity of AST by 20%. In comparison, aminoguanidine 1.0
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Enzyme Inhibition and Bioapplications
mM almost completely reversed negative effect of MGO and the AST activity of the sample
containing AST + MGO + AG was 86.2 and 76.6% of control sample activity after 120 and
240 min, respectively. The effect of Trolox 2.5 mM was slightly higher than that of NtBHA 5
mM. The AST activity was by 15.1 and 13.7% higher in the presence of Trolox after 120 and
240 min, respectively.
Fig. 14. Direct effect of NtBHA (panel A) and PBN (panel B) on AST catalytic activity. AST
(0.5 µg/ml) was incubated with or without methylglyoxal (0.5 mM) in 0.1 M sodium
phosphate buffer (pH 7.4) at 37 °C in the presence or absence of NtBHA (1-10 mM) or PBN
(1-10 mM). Catalytic activity of AST was expressed as percentage of each sample activity at
the time 0, which was 100% ± S.D. (%). Every point represents an average of eighteen (AST
and AST + MGO) or six independent samples (*data with P<0.01, Student’s t-test)
Fig. 15. Antiglycating activity of NtBHA (panel A) and PBN (panel B) towards
methylglyoxal-induced deactivation of AST. AST (0.5 µg/ml) was incubated with or
without methylglyoxal (0.5 mM) in 0.1 M sodium phosphate buffer (pH 7.4) at 37 °C in the
presence or absence of NtBHA (1-10 mM) or PBN (1-10 mM). Catalytic activity of AST was
expressed as percentage of each sample activity at the time 0, which was 100% ± S.D. (%).
Every point represents an average of eighteen (AST and AST + MGO) or six independent
samples (*data with P<0.01, Student’s t-test)
3.2 Fluorescence measurements
The inhibition of MGO-mediated protein glycation by several antioxidants was determined
by measuring of AGEs with fluorescent properties. Figure 16B shows formation of
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
103
argpyrimidine and the effect of NtBHA on this process. Sample containing AST + MGO
exerted 12.5 times higher fluorescence intensity than the control sample without MGO after
7 days of incubation. NtBHA caused statistically significant decrease in the formation of
argpyrimidine during incubation (inhibition by 42.3–83.1%). The most remarkable decline in
argpyrimidine formation was observed at 10 mM concentration of NtBHA, which exhibited
inhibition by 83.1%. Effect of PBN on the argpyrimidine formation was less remarkable
(inhibition by 35.2–55.8%) but still highly significant (Fig. 16A). The influence of AG 1.0 mM
was well-pronounced (93.2%), while the effect of Trolox 2.5 mM was much weaker (53.2%)
and comparable to the activity of NtBHA 1 mM and PBN 1-10 mM.
The effect of NtBHA on the formation of ‘‘non-specific’’ AGE products is presented in Fig.
16D. Methylglyoxal caused almost 13 fold increase in concentration of AGEs with fluorescent
properties compared to the control sample (AST alone) after 7 days of incubation. Positive
effect of NtBHA reached almost the same extent as in the case of argpyrimidine formation
with statistically significant inhibition of glycation (34.3–76.6%). Little bit lower rate of
inhibition (8.9–48.2%) was obtained also with PBN (Fig. 16C). Aminoguanidine and Trolox
showed 88.7 and 44.4% suppressing effect on AGEs generation, respectively.
As for fluorescence measurement, control sample showed stable but not negligible
fluorescence, since the start of the experiment. Most of this fluorescence is probably
constituted by general fluorescence properties of this protein. Presence of pyridoxal-5’phosphate coenzyme in the molecule of AST also contributes to basal fluorescence of the
enzyme. Results of fluorescence measurements clearly show an inhibiting effect of NtBHA
and PBN on the formation of AGE products. NtBHA was also quite effective in the
inhibition of argpyrimidine generation. Apart from these findings, the use of fluorescence
method for evaluation of protein glycation is limited by its imprecision. The measurement of
some well-identified AGEs (e.g., pentosidine and carboxymethyllysine) by techniques as
HPLC or ELISA could give more precise information on this matter (Boušová et al., 2009).
3.3 Determination of primary amino groups
Following incubation of AST with methylglyoxal, there was a decrease in amine content
compared to control (Table 1). Unmodified AST exhibited 25.2 ± 0.4 nmol NH2/mol AST
versus 13.9 ± 1.1 nmol NH2/mol AST for sample containing AST + MGO (P<0.001, using
Student’s t-test). This difference represents a 45% decrease in amine content due to chemical
modification of the primary amines (α-amino group of N-terminal amino acids and ε-amino
group of Lys residues). Native AST in the dimer form contains 40 primary amines (38 Lys
residues and 2 N-terminal amino acids), suggesting that approximately 18 amines were
modified by methylglyoxal. Modification of primary amino group of Lys258 in AST
molecule by MGO may be also responsible for the loss of its catalytic activity, because this
Lys residue binds coenzyme PLP in the active centre of AST and thus directly participates in
the enzymatic catalysis.
PBN as well as NtBHA significantly protected AST against the loss of primary amino groups
induced by MGO. Their effect was concentration-dependent and more pronounced in the
case of NtBHA. Sample containing AST + MGO + NtBHA 10 mM exhibited 21.8 ± 0.7 nmol
NH2/mol AST (P<0.001, using Student’s t-test) suggesting that about 5 amines were
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Enzyme Inhibition and Bioapplications
modified by MGO. In the sample containing AST + MGO + PBN 10 mM was found 19.8 ±
1.2 nmol NH2/mol AST (P<0.001, using Student’s t-test), which means that approximately 9
amines were lost. This result is comparable to the effect of NtBHA at 1 mM concentration.
Moreover, AG 1 mM exerted the same effect as NtBHA 10 mM. In the sample containing
AST + MGO + AG was detected 22.0 ± 0.4 nmol NH2/mol AST suggesting that 5 amines
was lost in its presence.
Fig. 16. Formation of argpyrimidine (panel A and B) and fluorescent AGEs (panel C and D).
AST (0.5 mg/ml) was incubated with or without methylglyoxal (0.5 mM) in 0.1 M sodium
phosphate buffer (pH 7.4) at 37 °C in the presence or absence of PBN (0.5-10 mM), NtBHA
(0.5–10 mM), aminoguanidine (1 mM) or Trolox (2.5 mM) for 7 days. Aliquots of samples
were taken on days 0 and 7 and stored frozen at -20 °C. Fluorescence of samples was
measured at specific excitation and emission wavelengths (λex/λem) corresponding to
argpyrimidine (335/385 nm) and AGEs (330/410 nm) versus the unincubated blanks. Data
of fluorescence were expressed in relative fluorescence units ± S.D. Every point represents
an average of two independent experiments (6 samples). Groups with different letters are
significantly different (P<0.01, Student’s t-test).
To determine whether the changes in fluorescence of argpyrimidine observed in the samples
containing various concentrations of NtBHA (0.5–10 mM) and MGO 0.5 mM correlated with
the loss of primary amino groups in these samples, the fluorescence intensity was plotted as
a function of amine content (data not shown). The decrease in fluorescence emission
(λex/λem = 320/380 nm) varied directly with the loss of primary amino groups (r = 0.963,
P<0.019). Changes in fluorescence of argpyrimidine in these samples also correlated well
105
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
with the formation of fluorescent AGEs (r = 0.991, P<0.005). In addition, the amine content
measured in samples containing AST + MGO + NtBHA (0.5-10 mM) was plotted as a
function of AGEs fluorescence, indicating that MGO-induced formation of AGEs was
directly proportional to an irreversible loss of primary amino groups in AST molecule (r =
0.946, P<0.027). Similar results were obtained also in samples containing AST + MGO + PBN
(0.5-10 mM).
Sample
AST
AST + MGO 0.5 mM
AST + MGO + PBN 0.5 mM
AST + MGO + PBN 1 mM
AST + MGO + PBN 5 mM
AST + MGO + PBN 10 mM
AST + MGO + NtBHA 0.5 mM
AST + MGO + NtBHA 1 mM
AST + MGO + NtBHA 5 mM
AST + MGO + NtBHA 10 mM
AST + MGO + AG 1 mM
a,b,c,d,e,f
Amine content
Number of primary NH2
(nmol NH2/mol AST)
remaining
modified
25.2 ± 0.4a
13.9 ± 1.1b
17.9 ± 0.9c
18.2 ± 1.1cd
18.6 ± 1.7cd
19.8 ± 1.2d
17.3 ± 0.4c
19.5 ± 0.4d
20.8 ± 0.8de
21.8 ± 0.7ef
22.0 ± 0.4f
40
22
28
29
30
31
27
31
32
35
35
0
18
12
11
10
9
13
9
8
5
5
Groups with different letters vary significantly (P<0.05, Student’s t-test)
Table 1. Effect of PBN, NtBHA and AG on the changes in AST primary amine content
induced by MGO 0.5 mM
3.4 Effect of glycation on molecular charge of AST
Native PAGE was run several times, and the representative native PAGE gel is presented in
Fig. 17. Mobility of MGO-modified protein to the positive pole significantly increased (by
41%) after 7 days of incubation compared to the mobility of control sample (AST alone). This
result indicates the progressive loss of the positive charge in the MGO-modified AST during
the glycation reaction. NtBHA showed concentration-dependent protective effect against
changes in AST molecular change induced by MGO, when the relative mobility of sample
containing AST + MGO + NtBHA 10 mM was increased only by 9.2% compared to the
mobility of control. The enzyme incubated in the presence of both MGO and PBN showed a
smaller rise in mobility, up to 21.6% in the case of PBN 10 mM. The effect of this compound
on the protein electrophoretic mobility ranged from 21.6 to 26.3%. Aminoguanidine 1 mM
completely reversed effect of MGO and the relative mobility of sample containing AST +
MGO + AG was only slightly increased (by 0.35%) against the mobility of control, whereas
Trolox 2.5 mM showed similar effect on molecular charge of AST as PBN, i.e., the mobility
was increased by 27% compared to control sample (data not shown). These data indicated
that the molecule of enzyme became more anionic due to glycation and that PBN as well as
NtBHA had significant inhibitory effect on the middle stage of glycation process.
106
Enzyme Inhibition and Bioapplications
Fig. 17. Protective effect of NtBHA and PBN on changes in molecular change of AST caused
by MGO-induced glycation. AST (0.5 mg/ml) was incubated with or without MGO (0.5
mM) and NtBHA (0.5-10 mM) or PBN (0.5-10 mM) in sodium phosphate buffer (0.1 M, pH
7.4) at 37 °C for 7 days and then subjected to native PAGE. Proteins were visualized by
Coomassie Blue G250. Gels were scanned and Rf was obtained using Quantity One software
Methylglyoxal-induced chemical modifications led to a change in molecular charge of AST,
which became more anionic as revealed by native PAGE. These results indicate the
progressive loss of the positive charge in the glycation-modified AST molecule, which is
caused by the irreversible modification of Arg and Lys residues (Kang, 2006; Nagai et al.,
2000) as was confirmed by determination of amine content. Both PBN and NtBHA partially
protected native AST against glycation by MGO. The antiglycation activity was more
pronounced in the case of NtBHA mainly at higher concentrations tested. The antiglycation
activity of NtBHA 10 mM was a little bit lower than that of AG 1 mM.
3.5 SDS-PAGE and western blotting
The ability of aggregation and cross-link formation of tested antioxidants was determined
by SDS-PAGE under denaturing conditions (Fig. 18). MGO readily reacts with lysine and
arginine residues to produce high molecular weight protein products. Incubation of AST
with MGO 0.5 mM at 37 °C for 7 days resulted in the formation of protein aggregates with
molecular weight about 85, 107, and 145 kDa corresponding to protein dimer, trimer, and
tetramer, respectively. No presence of protein dimer and tetramer, and lower concentration
of protein trimer were observed in samples containing AST alone (lane 2), AST + NtBHA 10
mM (lane 7), and AST + AG 1 mM (data not shown). Also lower concentrations of NtBHA
were able to partially protect formation of protein tetramer, although they had no effect on
formation of protein dimer and trimer. On the other hand, PBN as well as Trolox were not
able to prevent formation of protein cross-links and high molecular weight aggregates.
Additional bands with molecular weight 20–35, 57 and 63 kDa were constituted of several
contaminating proteins present in commercial preparation (Fig. 18).
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
107
Western blotting with specific antibody against advanced glycation end products derived
from MGO (anti-MGO [3C]) was used to confirm formation of protein aggregates as a result
of MGO activity. The presence of high molecular weight protein cross-links in samples
containing AST + MGO, AST + MGO + NtBHA, and AST + MGO + PBN was observed (data
not shown). These protein aggregates had molecular weight about 85, 107, and 145 kDa
corresponding to AST dimer, trimer, and tetramer, respectively. Quantitative differences
between bands of samples with and without PBN or Trolox were not observed. These
compounds are not able to prevent formation of protein cross-links. On the other hand,
some reduction in the amount of AST tetramer was observed in samples containing NtBHA.
These results suggest that NtBHA possesses, at least in part, antiglycation properties.
Nevertheless, aminoguanidine 1 mM completely inhibited formation of protein aggregates,
since no bands of AST dimer, trimer or tetramer were present.
The electrophoretic techniques confirmed the results obtained by other methods; i.e., changes
in protein molecule caused by the presence of methylglyoxal and positive antiglycating effect
of NtBHA. Methylglyoxal-induced chemical modifications led to a change in molecular charge
of AST, which became more anionic as revealed by native PAGE. The SDS-PAGE and
subsequent western blotting clearly showed formation of protein cross-links with higher
molecular weight than native enzyme. NtBHA partially protected native AST from glycation
by MGO and also exhibited mild anti-cross-linking activity.
Fig. 18. Formation of protein cross-links on reaction of AST with methylglyoxal. AST
(0.5 mg/ml) was incubated with or without methylglyoxal (0.5 mM) in sodium phosphate
buffer (0.1 M, pH 7.4) at 37 °C in the presence or absence of N-tert-butyl hydroxylamine
(0.5–10 mM) for 7 days and then subjected to SDS-PAGE. Electrophoretic separation was
performed on 4% stacking and 10% resolving polyacrylamide gels under reducing
conditions. Bands were visualized with silver staining. Each lane was loaded with 4 µg of
protein. MM = Mw marker; AST = aspartate aminotransferase; MGO = methylglyoxal;
NtBHA = N-tert-butyl hydroxylamine
108
Enzyme Inhibition and Bioapplications
Modification of proteins caused by methylglyoxal can be accompanied by formation of free
radicals. Lee et al. (1998) identified three types of free radical species in samples containing
methylglyoxal and bovine serum albumin by electron spin resonance spectroscopy. These
radicals (methylglyoxal dialkylimine radical cation, methylglyoxal radical anion, and
superoxide anion radical) were formed by direct 1-electron transfer process. Scavenging
ability of NtBHA and PBN were already described (Lee et al., 2004; Atamna et al., 2001). It
can be assumed that the positive antiglycation activity of these compounds may be at least
partly attributed to their scavenging ability.
4. Conclusion
-
-
-
-
-
-
Catalytic activity, which is biologically the most important property of all enzymes, is
fully dependent on the native structure of the enzyme. Changes in the structure of
enzymes usually lead to the progressive loss of their catalytic activities. These changes
may be caused by reversible binding of various low-molecular inhibitors or by
irreversible modification. Such example of the irreversible change is non-enzymatic
glycation of proteins by various reducing monosaccharides or reactive α-dicarbonyl
compounds (e.g., MGO).
In our studies, aspartate aminotransferase (AST) was used in glycation studies as model
protein, which possesses catalytic properties.
Catalytic activity of aminotransferases in vitro has been found to be impaired by
glycating agents. The extent of this effect depends on the activity and concentration of
the agent, susceptibility of given enzyme to such modification as well as on the
duration of action (see Fig. 4, Fig. 5 and Fig. 8).
Among several glycating agents, effect of fructose is strong enough for investigation of
changing AST properties during long time of incubation. Nevertheless, methylglyoxal,
an intermediate of glycation process, is more reactive and permits to investigate the
process of AST glycation in vitro in shorter course of time.
Catalytic activity of AST as the protein function may serve as the most important
criterion of glycation effect. Beside this, molecular properties of purified AST and
character of glycoxidation permit using other methods of investigation of molecular
changes during the process, like fluorescence of advanced glycation end-products,
decrease in primary amino groups in the protein molecule, and protein cross-linking
and aggregation.
Several approaches of therapeutic intervention to the glycation process have been used
(e.g., reduction in deposition of already formed AGEs, inhibition of new AGEs
formation, and inhibition of the receptor for AGE).
In our own experiments, compounds with described antioxidant and potential
antiglycating activities have been studied. Among the compounds studied,
hydroxycitric acid, uric acid, and two mitochondrial antioxidants α-phenyl N-tert-butyl
nitrone and N-tert-butyl hydroxylamine had pronounced antiglycating activity against
protein glycation by methylglyoxal (hydroxycitric acid, PBN and NtBHA) and by
fructose (hydroxycitric and uric acids). On the other hand, flavonoids baicalin and
baicalein exerted overall negative influence on the catalytic activity of AST alone and in
the combination with fructose. Other studied compounds (i.e., ferulic, isoferulic, ocoumaric, and p-coumaric acids, arbutin and methylarbutin) showed no positive
antiglycation activity.
Non-Enzymatic Glycation of Aminotransferases and the Possibilities of Its Modulation
-
109
The goal of our research group is to participate in the search for compounds with
potential antiglycating activity with a perspective of their use as remedies against
diabetic complications.
5. Acknowledgment
This study was supported by the Charles University in Prague (Project SVV 263 004).
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5
Inhibition of Nitric Oxide Synthase Gene
Expression: In vivo Imaging Approaches of
Nitric Oxide with Multimodal Imaging
1Center
Rakesh Sharma1,2
of Nanomagnetics and Biotechnology, Florida State University, Tallahassee, FL
2Amity Institute of Nanotechnology, Amity University, NOIDA, U.P.
India
1. Introduction
Nitric oxide is an uncharged free radical abundant in nanomolar quantities and detected by
measuring nitric oxide synthase (NOS). Nitric oxide is a gas highly reactive, short lived free
radical generated enzymatically by NOS involved in diverse physiological (neurotransmission,
immune system) and pathophysiological (tumor progression) mechanisms. Nitric oxide is a
biological mediator for its role as EDRF (endothelial-derived relaxing factor) responsible for
the regulation of blood vessel relaxation and blood pressure maintenance [Labet et al. 2009]. In
recent years, inhibition of NOS gene expression has become a scientific interest to measure NO
in tissues and synthetic NOS/NO inhibitors have revolutionized molecular imaging of tissues.
Present chapter presents a journey from major mechanistic concepts to the detection of NO,
NOS and their multimodal bioimaging applications with limitations.
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Several genes involve in NOS enzyme for its synthase activity (NOS1, NOS2A, NOS3);
oxidoreductase activity (NOS1, NOS2A, NOS3, NQO1); as positive regulators(HSP90AB1
or HSPCB, INS); as negative regulators (DNCL1, GLA, IL10);other AKT1, ARG2, DDAH2,
DNCL1, EGFR, GCH1, GCHFR genes
NO diffuses freely across cell membranes. It is short lived but combines with metallic
aromatic ‘spin traps' to make stable compounds. NO acts in a paracrine or autocrine
manner influencing only cells near its point of synthesis.
NO is synthesized within cells by a flavocytochrome enzyme NO synthase (NOS). The
human contains 3 different NO synthases:
nNOS (NOS-1) is found in neurons (hence the "n"); iNOS (inducible NOS-2), triggered
by inflammatory cytokines found in macrophages; and eNOS (NOS-3): constitutively
distributed in the vascular endothelium lining the lumen of blood vessels, lung, and
platelets (called constitutive NOS or cNOS) [Perrier, et al. 2009]. All types of NOS
produce NO from arginine with the aid of molecular oxygen and NADPH as shown in
following redox reaction and Figure 1. Inhibition of NOS gene expression in cells may
detect NO [Nie et al.2008; Terashima et al. 2010].
NADH-diphorase stain the NOS expression to detect NO in tissues
NO is generated in vascular endothelium cells (NO plays role in the regulation of
vascular tone), peripheral and central neurons (NO acts as synaptic neuronal
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Enzyme Inhibition and Bioapplications
messenger) by three isoforms of NO synthase, endothelial, neuronal, and inducible
form [Perrier, et al. 2009, Claudette, et al. 2005]. Detection and in vivo monitoring of NO
is very difficult. Spin traps, fluorescent dyes, chemi/bioluminescent sensors detect and
image NO by ESR,NMR,PET,US,and optical methods.
Fig. 1. Redox potentials and direction of electron flow in nNOS enzyme action are shown.
The electron flow in the NOS dimer goes via NADPHFADFMN in the reductase of one
monomer to the haem iron in the oxygenase domain of a separate monomer. The redox
potentials are poised thermodynamically to make this occur. The potentials for the twoelectron oxidation of NADPH and the one-electron oxidations of FADH2, FMNH2 and ferric
haem are illustrated at the bottom of the Scheme, with the red arrows indicating the
direction of electron flow. Note that given the variety of redox couples and the closeness of
the FADH2 and FMNH2 potentials the detailed picture is more complex than that illustrated.
For example the FMNH+/FMN couple has a redox potential of -49 mV and is likely to
donate electrons to the high-potential ferric superoxide species to form the ferryl
intermediate. Reduction of the FMN back to FMNH+ would then require an electron from
FMNH+. Adapted from Alderton, et al. 2001.

Mapping the distribution of NO generation or formation of stable spin-trapping species at
different locations in tissues or organs by in vivo perfusion (3D visualization of NO
Inhibition of Nitric Oxide Synthase Gene Expression:
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117
structural-functional or conformational dynamics as switch OFF and ON) may allow us
to understand the real physiological function of NO gas or ionic form under different
tissue physiological conditions [Thatte et al. 2009]. Using in vivo nitric oxide direct
detection by stabilizing NO with suitable spin-trapping reagents is a challenge to estimate
the in vivo NO concentration by MRI techniques [Hong et al. 2009; Sari-Sarraf, et al. 2009;
Liu, et al. 2009; Ny et al. 2008; Fujii, et al, 2007; Vandsburger, et al.2007; Samouilov et al.
2007; Flögel, et al. 2007; Bobko, et al. 2005; Day, et al. 2005; Waller, et al. 2005; Sirmatel, et
al. 2007; Itoh, et al. 2004; Berliner, et al. 2004; Haga,et al. 2003; Li, et al. 2003; Hsiao, et al.
2008; Kuppusamy, et al. 2001; Fichtlscherer, et al. 2000; Fujii,et al.1999; Fujii, et al. 2002].
Other nitric oxide bioimaging technique is flourimetry using fluorescent biomarkers in
EPR spectroscopy [Kojima, et al.2001; Fuji, et al.1997; Yoshimura, et al. 1996]. Our
immediate focus is to highlight the existing multimodal mechanisms of NO sensitive
MRI/EPR signal generation using NOS enzyme expression inhibitors and biosensors to
map cellular events.
1.1 Nitric oxide as a second messenger in cellular signaling
Nitric oxide signal transduction through induced-nitric-oxide modifications relies on the
system of Cys-based posttranslational modifications. Accordingly, S-nitrosylation of
proteins plays an essential role in downstream cascades (Do et al., 1996). Nitric oxide exerts
an ubiquitous influence on cellular signaling in large part by means of Snitrosylation/denitrosylation of protein cysteine residues. S-NO undergo a regulated posttranslational protein modification specific to NO-derived effects. These NO-dependent
modifications influence protein activity, protein-protein interactions, and protein location. Snitrosylation thus serves as the prototypical redox-based signal (Janssen-Heininger et al.,
2008). S-Nitrosylation has been implicated in transmitting signals downstream of all classes
of receptors, including G-protein-coupled receptor (GPCR), receptor tyrosine kinase, tumor
necrosis factor, Toll-receptors, and glutaminergic receptors, acting locally within subcellular
signaling domains as well conveying signals from the cell surface to intracellular
compartments, including the mitochondria and the nucleus (Janssen-Heininger et al., 2008).
Cell signaling through S-nitrosylation is useful tool in signaling transduction for:
1.
2.
3.
4.
5.
Temporal regulation of response through a rapid and controlled stimulation;
The existence of motifs within proteins that provides S-nitrosylation specificity;
Colocalization of target proteins with a source of NO;
Reversibility of protein S-nitrosylation;
Enzymatic control of S-nitrosylation through the action of S-nitrosoglutathione
reductase (Janssen-Heininger et al., 2008).
1.2 Why inhibition of NOS expression as bioimaging of NO technique
Mapping the distribution of NO generation at different locations in different tissues and
organs by in vivo perfusion reveals the physiological function of NO (gas or ionic form) in
different tissue physiological conditions [Thatte, et al.2009]. Major nitric oxide imaging
techniques utilize mapping NO in tissue using NO specific imaging contrast agents sensitive
to fluorescence, magnetic resonance and electron spin resonance. Imaging in vivo physical
properties of tissue cells such as cell calcium signaling and NO-biomarker is new way by
proton magnetic resonance techniques to achieve nanomolar range of nitric oxide mapping
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Enzyme Inhibition and Bioapplications
without any toxic effects[Thatte, et al.2009, Hong et al.2009]. Fluorescent nitric oxide
cheletropic traps are currently available choices in nitric oxide imaging but all of these have
pitfalls of causing neurotoxicity [Reif et al.2009]. In this chapter, we display evidence of the
NO sensitive fluorescent probes as cell calcium signaling indicator and possibility of NO
specific perfusion MRI tool to visualize physiological nanomolar dynamics of NO in living
cells and tissues up to the detection limit of 0.1 nM. The cell signaling indicators such as
intracellular calcium revealed that ~1 nM of NO was enough to detect apoptosis events such
as caspase 3 activation [Li et al.2009].
Fig. 2. Domain structure of human nNOS, eNOS and iNOS(on left), Overall reaction
catalysed and cofactors of NOS(on right). Adapted from Alderton, et al. 2001.
1.2.1 Feasibility of NOS expression inhibitors as imaging Contrast Agents
NO is a gaseous and highly active neuronal messenger, short lived free radical generated
enzymatically by NOS in the brain[ Perrier et al.2009]. NOS is a flavocytochrome that is
constitutively distributed in the vascular endothelium, brain, lung, and platelets (called
constitutive NOS or cNOS), but is also found as an inducible form (called inducible NOS or
iNOS) in many cells and organs, triggered by several factors such as inflammatory cytokines
[Claudette et al.2009]. However, NOS is abundant in three isoforms cNOS, iNOS, eNOS as
shown in Figure 2. NOS undergo dimerization in presence of BH4 or heme or arginine binding
sites. Different roles of flabin, heme, pterin cofactors in NOS are described in detail [Alderton,
et al. 2001, Matter et a. 2004]. Authors reviewed NOS activity regulation by calmodulin,
phosphorylation, protein inhibitors of NOS(PIN), heat shock protein 90(Hsp 90),
Inhibition of Nitric Oxide Synthase Gene Expression:
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119
myristoylation, palmitoylation, caveolin, at different domain located in NOS or spilce variants:
nNOSβ and nNOSγ, nNOSµ, nNOS-2, iNOS/eNOS splice variants. However, cross-activities
of all three isoforms of NOS and their dependence on calcium, common locations have put
challenge to specificity and gave way to a new way of specific NO/NOS inhibitor compounds.
In recent years, NOS gene expression and its regulation have got attention for two reasons:
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NOS gene expression can be stained, imaged for multimodal molecular imaging
The NOS inhibitors inhibit NOS as potent anti-inflammatory agents in recent years.
Visualization of NOS inhibition is believed to serve as in vivo or in vitro biomarkers of
the proinflammatory status of NOS inhibitors or in vivo proinflammation bioimaging of
tissues.
Some examples of NOS ihibitors are given below.
1.2.2 NOS Inhibitors
NOS inhibition occurs at arginine site, tetrahydrobiopterin site, pteridine site, and heme
domain by partially selective or highly selective inhibitors [Alderton, et al. 2001, Matter et a.
2004]. Excellent information of binding site interactions of NOS with relationships at
different sites is available [Alderton, et al. 2001, Matter et al. 2004]. To understand better the
NOS inhibitor structural-activity relationship, X-Ray analysis, 3D QUSAR, comparative
molecular field analysis (CoMFA) analysis, GRID/PCA interpretations have enhanced the
scope of NOS in pharmacology as shown in Figures 3,4,5 [Matter et al.2004]. There are array
of NOS inhibitors described in the literature as drug testing tools. Table 1 shows efficacy of
some of these in inhibiting the three human NOS isoforms.. Of these the most widely used
have been l-NMMA, l-NNA and its methyl ester prodrug (NG-nitro-l-arginine methyl ester,
`l-NAME' and aminoguanidine. However, inhibitors show pitfalls on selectivity to NOS,
types of interactions with iNOS and nNOS isoforms. Selective NOS inhibitors may be
selective in the physiological range (l-arginine concentration etc). Inhibitor agents with 1050-fold selectivity are useful as `partially selective inhibitors.
IC50 (µM)
inhibitor
L-NNA*
L-NMMA
7-NI*
ARL17477*
Aminoguanidine*
L-NIL
1400W
GW273629
GW274150
iNOS
nNOS
eNOS
3.1
6.6
9.7
0.33
31
1.6
0.23.
8.0.
1.4.
0.29
4.9
8.3
0.07
170
37
7.3
630
145
0.35
3.5
11.8
1.6
330
49
1000
1000
466
Selectivity (fold)
iNOS vs
iNOS vs nNOS vs
nNOS
eNOS
eNOS
0.09
0.11
1.2
0.7
0.5
0.7
0.9
1.2
1.4
0.2
5
23.
5.5
11
1.9
23.
49.
1.3
32.
4000*.
130*
78.
125*.
1.6*
104.
333
3.2
The data shown are for inhibition of the human NOS isoforms in the presence of 30 µM L-arginine at 37
°C over 15 min after a 15 min pre-incubation with inhibitor under turnover. The data serves as
progressive inhibitory mechanisms for the NOS assay. Data are from Young et al. 2000.
Table 1. Selectivity of inhibitors of NOSs is compared for different inhibitors
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Enzyme Inhibition and Bioapplications
Fig. 3. Contour maps for NOS-I comparative molecular field analysis (CoMFA) analysis with
a 4-amino-pteridine inhibitor.A: Steric contour map, green contours indicate sterically
favored regions, yellow contours indicate unfavored areas. B: Samethan, A: with NOS-I
binding site. C: Electrostatic contour map, blue contours refer to regions, where negatively
charged substituents are unfavorable, red contours indicate regions, where negatively
charged substituents are favorable. D: Same as C with NOS-I binding site. Reproduced with
permission from Matter et al. 2004.
Fig. 4. A: Score plot from GRID/principal componentanalysis (PCA) based on13
conformations and 3NOS isoforms. B: SuperpositionofNOS-III (1nse, blue) toNOS-IX-ray
(magenta) andhumanbrainNOS-I (homology, purple).C:Scoreplot fromGRID/CPCA for
hydrophobic interactions (GRID dry probe). D:GRID/CPCA differential plots highlighting
differences between NOS-I/NOS-II and NOS-I/NOS-III.Favorable interactions
toachieveNOS-I isoformselectivities are shownincyancontours, unfavorable interactions are
displayed in yellow with respect to H4Bipfromthe NOS-III1nse X-ray structure. Reproduced
with permission from reference [Matter et al. 2004]
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The mechanistic nature of three human NOS isoforms was expressed in the baculovirus
expression system, and cell lysates as the enzyme source. iNOS potency and selectivity
underestimates progressive inhibition of iNOS but not eNOS or nNOS; e.g. for 1400W the
steady-state values of iNOS IC50 and selectivity have been estimated to be 0.1 µM, 250fold (versus nNOS) and 5000-fold (versus eNOS) as reported by Young, et al. 2000. Highly
selective compounds of over 50- or 100-fold selectivity, inhibit the NOS activity of a single
isoform without affecting others. They have potential as selective therapeutic agents
without side effects. Recently, 7-nitroindazole (7-NI) was reported as non selective nNOS
inhibitor, of isolated NOS enzyme (Table 1). 7-NI showed inhibition of nNOS independent
of increases in blood pressure but showed eNOS-dependent celltype specificity (neuronal
verus endothelial), intracellular BH4 concentration, or depending on specific cellular
transport or metabolism [Handy, et al. 1998]. All three NOS isoforms can be expressed in
neurons and both eNOS and iNOS in endothelial cells. Cell-type specificity is clearly a
very distinct phenomenon from isoform selectivity. Other pitfall is particular dose. For
example, in humans, the non-selective NOS inhibitor l-NMMA causes a five-fold increase
in vascular resistance with only a 10% change in blood pressure, because of reflex
decreases in cardiac output 185±187 [Ross et al. 1998] Suppression of inhibitor-induced
plasma nitrate (mediated predominantly by iNOS) and no effects on blood pressure has
led to inhibitors as non selective, e.g. S-ethylisothiourea [Raman, et al. 1998]. Selectivity
for iNOS versus eNOS distinct enzyme targets is another pitfall. Time-dependent NOS
inhibition in assessing efficacy and selectivity of NOS inhibitors were first reviewed by
Bryk and Wolff, 1999.
1.2.3 NOS inhibitor interactions with the NOS enzymes
Inhibitors of NOS have been described which interact with the NOS enzymes in a variety of
ways: different sites, differing time- and substrate-dependence, and mechanism of
inhibition.
L-Arginine site inhibitors identified so far as competitive with the substrate L-arginine
binding at the arginine-binding site inhibitors (aminoguanidine, S-ethylisothiourea,
thiocitrulline) [Raman, et al.1998].
Mechanism-based inhibitors of iNOS require active enzyme and NADPH substrate to
permit inhibition to proceed through multiple enzyme covalent modification by a
complex formation pathway: EI-EI* complex where E is iNOS, I is the inhibitor, EI is the
initial non-covalent complex and EI* is a modified complex, either with a conformational
change to tight binding or with covalent changes to the enzyme, inhibitor or both. The
interactions of some of these pterin-site inhibitors with NOS reveal unexpected
complexity. An example of a mechanism based iNOS inhibition was cited by
aminoguanidine [197±199] and by the acetamidine inhibitors N-α-iminoethyl-l-ornithine
(l-NIO) and N'-iminoethyl-lysine (l-NIL) [200±203], GW273629 (S-[2-[(1-iminoethyl)amino]ethyl]-4,4-dioxo-l-cysteine) and GW274150 (S-[2-[(1-iminoethyl)amino]ethyl]-lhomocysteine) (see Table 1). Heme-binding inhibitors have been shown to bind with
each NOS monomer, one to the haem iron and one to the arginine-binding region (Glu ) in
competition with CaM. These compounds affect the assembly of iNOS monomers into
active dimer inhibiting the dimerization [Sennequier, et al. 1999]. A class of substituted
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Enzyme Inhibition and Bioapplications
pyrimidine inidazoles have been identified which inhibit dimerization of iNOS during its
synthesis and assembly.
1.2.4 Flavoprotein and CaM inhibitors
A range of flavoproteins (e.g. diphenylene iodonium) or CaM (e.g. trifluoperazine) has been
shown to inhibit NOS. NOS inhibitors display selectivity of their isoforms as partially
selective, highly selective.
Partially and highly isoform-selective NOS inhibitors
Identification of selective inhibitors of iNOS and nNOS "100-fold selectivity for iNOS versus
eNOS are reported [Anon, 1999a, 1999b]. Partially-selective nNOS inhibitor amino acids for
nNOS versus eNOS and iNOS were reported. For example, S-ethyl- and S-methyl-1thiocitrulline, vinyl-l-NIO showed timedependent inhibition of nNOS with significant
selectivities versus isolated eNOS and iNOS enzymes [Babu and Griffith, 1998]. Other
partially-selective iNOS inhibitors such as acetamidine-containing analogues of arginine, lNIO and l-NIL have been widely used to probe the effects of iNOS inhibition. For example,
some 2-iminohomopiperidines and 2-iminopyrrolidines with high (100±900-fold) selectivity
for iNOS versus eNOS, but similar potency was observed on iNOS and nNOS (1±13-fold
selectivity), with dual action iNOS-nNOS inhibitors.
Highly-selective iNOS inhibitors
The `highly selective ' iNOS inhibitors versus eNOS are mostly bis-isothioureas. Of these, S,S[1,3-phenylene bis-(1,2-ethanediyl)bis-thiourea (`PBITU') is an l-arginine-competitive, rapidly
reversible inhibitor of human iNOS with a Ki of 47 nM, and a selectivity (in Ki terms) of 190fold versus eNOS showing substrate-binding sites of full-length human iNOS and eNOS in
solution or in cells and tissues. Inhibition of human iNOS by 1400W was competitive with larginine, NADPH-dependent either an irreversible, or reversible. Mechanism-based inhibitor
action was reported as Kd value %7 nM and steady-state selectivity against eNOS and nNOS of
5000 and 250-fold respective effects on vascular leakage [Lazlo, et al.1997]. GW273629 and
GW274150 are two novel NOS inhibitors for iNOS versus both eNOS and nNOS. Both are
sulphur-substituted acetamidine amino acids acting in competition with l-arginine as
NADPH-dependent, whereas the inhibition of human eNOS and nNOS is rapidly reversible.
The heme-binding substituted pyrimidine imidazoles inhibit assembly of active dimeric iNOS
during its synthesis. It will be interesting to see what the pharmacology and utility of such
compounds will be, and whether other compound series are discovered with this NOS
inhibition mechanism of action.
1.3 The pharmacological inhibition of inducible nitric-oxide synthase (iNOS) gene
expression
Presently, inhibition of NOS gene expression approach is used in testing inhibitors of
pharmacological value. In past, inhibition of NOS was ideal assay to measure less stable NO
in tissues but less specificity of NOS was discouragement and spin trap agents have
emerged as a new approach of detection and measurement of NO in both in vivo and in
vitro assays and bioimaging. Some examples of NOS inhibitors are cited in following
section.
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2.
3.
4.
5.
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AMP Kinase protein kinase induced inhibition of inducible nitric-oxide synthase (iNOS)
Inducible NOS inhibition in endotoxic shock in chronic inflammatory states was
observed in several cell types (myocytes, adipocytes, macrophages) and primarily
resulted from post-transcriptional regulation of the iNOS protein. Best example is
inhibition of inducible nitric oxide synthase by activators of AMP activated protein
kinase to explain a new mechanism of insulin sensitizing drug action [Pilon, et al.2004]..
Inflammatory cytokines and LPS trigger the iNOS transcription through a complex
network of intracellular pathways including NF-κB, Janus kinase/signal transducers
and activators of transcription, and mitogen-activated AMPK protein kinase by
reducing the transcription of iNOS and mRNA expression [Blanchette et al. 2003].
AMPK switches off protein synthesis either through suppression of the mTOR-p70S6
kinase pathway or by direct activation of eukaryotic elongation factor 2 kinase,
resulting in the phosphorylation and inactivation of eukaryotic elongation factor 2
[Horman et al. 2002]. AMPK reduces iNOS protein content by promoting its
ubiquitination, required for targeting iNOS through the proteasome proteolysis
pathway [Kolodziejski et al.2002].
Expression of exogenous Kalirin in pituitary cells dramatically reduces iNOS inhibition
of ACTH secretion. Kalirin inhibits iNOS activity by affecting iNOS homodimerization,
which is required for iNOS activity. Thus Kalirin may play a neuroprotective role
during inflammation of the central nervous system by inhibiting iNOS activity
[Ratovitski, et al. 1999].
N5-(Iminoalkyl)- and N5-(Iminoalkenyl)-ornithines (VNIO) and several L-VNIO analogs
showed minor structural changes to produce inhibitors either iNOS-selective or
nonselective [Bretscher et al. 2003]. Furthermore, derivatives having a methyl group
added to the butenyl moiety of L-VNIO and L-VNIO derivatives display slow-on, slowoff kinetics rather than irreversible inactivation. These results elucidate isoformselective inhibition by L-VNIO and may provide information useful in rational design
of isoform-selective inhibitors. [Bretscher, et al. 2003]
Constitutive and inducible isoforms of NOS are inhibited by S-alkyl-L-thiocitrullines
with n-alkyl groups of any one carbon. The NOS inhibition is reversible, stereoselective,
and competitive with L-arginine[Narayanan, et al. 1995].
Autoinhibition of endothelial NOS was reported by presence of an electron transfer
control element in the NOS. [Nishida, et al. 1999]. Investigators examined the role of
the insert in its native protein context by deleting the insert from both wild-type
eNOS and from chimeras obtained by swapping the reductase domains of the three
NOS isoforms. The Ca2+ concentrations required to activate the enzymes decrease
significantly when the insert is deleted, consistent with suppression of autoinhibition.
Furthermore, removal of the insert greatly enhances the maximal activity of wild-type
eNOS, the least active of the three isoforms. Despite the correlation between
reductase and overall enzymatic activity for the wild-type and chimeric NOS
proteins, the loop-free eNOS still requires CaM to synthesize zNO. However, the
reductive activity of the CaM-free, loop-deleted eNOS is enhanced significantly over
that of CaM-free wild-type eNOS and approaches the same level as that of
CaMbound wild-type eNOS. Thus, the inhibitory effect of the loop on both the eNOS
reductase and zNO-synthesizing activities may have an origin distinct from the loop’s
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6.
7.
Enzyme Inhibition and Bioapplications
inhibitory effects on the binding of CaM and the concomitant activation of the
reductase and zNO-synthesizing activities.
A mechanism was reported as Ca2+ triggers cross-talk signal transduction between
CaM kinase and NO and CaM-K IIa phosphorylating nNOS on Ser847, which in turn
decreases the gaseous second messenger NO in neuronal cells by
Calcium/Calmodulin-dependent Protein Kinase IIa in NG108-15 neuronal Cells.
[Komeima, et al. 2000].
Nitric oxide (NO) is moderately produced under control of iNOS gene expression. In
recent years, scientists solved the problem of visualizing very unstable NO by mapping
iNOS inhibition using gene expression array, in vivo nitric oxide direct detection (by
stabilizing NO with suitable spin-trapping reagents), bioluminesence and MRI
techniques to estimate in vivo NO concentration [Hong et al. 2009]. New approaches are
in the direction of multimodal bioimaging iNOS gene expression and NO bioimaging as
biomarkers. Success depends on NOS gene sensitive MR relaxation signal in cells and
specificity to inflammation [Nie et al.2008; Terashima et al. 2010].
2. NO inhibitors in imaging
Major nitric oxide imaging techniques utilize mapping NO in tissue using NO specific
imaging contrast agents sensitive to fluorescence, magnetic resonance and electron spin
resonance. Recently, focus is diverted towards imaging in vivo physical properties of tissue
cells such as cell calcium signaling and NO-biomarker based proton magnetic resonance
techniques to achieve nanomolar range of nitric oxide mapping without any toxic
effect. Fluorescent nitric oxide cheletropic traps are currently available choices in nitric oxide
imaging but all of these have pitfalls of causing neurotoxicity [Reif, et al.2009]. In following
sections, we put evidence of the NO sensitive fluorescent probes as cell calcium signaling
indicator in tissues and possibility of NO specific perfusion MRI tool to visualize
physiological nanomolar dynamics of NO in living cells up to the detection limit of 0.1 nM.
The cell signaling indicators such as intracellular calcium revealed that ~1 nM of NO was
enough to detect apoptosis events such as caspase-3 activation [Green, 1998, Nagata, 1997,
Kwon et al.2009]. Furthermore, the possibility of superparamagnetic iron oxide nanoparticle
bound complexes serve as MRI imaging contrast agents based on dephasing contrast. The
iron oxide chelates are still in active evaluation phase to test their toxicity. The nanomolar
range of basal endothelial NO appears to be fundamental to vascular homeostasis, hypoxia,
apoptosis and inflammation.


different contrast mechanisms and contrast characteristics of known nitrosyl-iron
complexes display possibility of potential multimodal MRI and EPR probes specific to
NO with examples of dithiacarbamates and Fe(MGD)2 complexs in different
applications.
fMRI detects the interaction of paramagnetic species with NO in blood but possibility is
still controversial. We review different applications of NO bioimaging.
2.1 Dithiacarbamates, LPS and MGD complexes for bioimaging of NO
In following sections we highlight the existing mechanisms and description to NO sensitive
signal generation. Existing mechanisms of NO sensitive signal generation are explored by
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MRI and fluorescence arising out from paramagnetic metals, dithiacarbamates or
lipopolysaccharides complexes. The mechanisms depend on three approaches:



Use of paramagnetic metals (SPIO) in MRI and dithiacarbamates (DTC) or
lipopolysaccharides (LPS) complexes sensitive to MRI and fluorescence effects;
Use of cytochrome proteins sensitive to EPR effect;
Use of NO synthase inhibitors to measure the reduced NO concentrations for
fluorometry and blood oxygen sensitivity to reduced NO concentrations.
The following sections are focused on dithiacarbamates in fluorometry and less known
imaging contrast agents in MRI to image nitric oxide in tissues.
The first evidence of dithiacarbamates (DTC) reported them as electron Fe(II)-chelate spin
trap agents. Examples are N-methyl-D-glucamine dithiocarbamate (MGD), (MGD)2-Fe(II)NO and NO-Fe-DTC metal complexes as multimodal imaging agents. These were initially
verified for EPR with possibility of visualizing the radical distribution by MR images
[Kubrina, et al.1992]. The (MGD)2-Fe(II)-NO complex enhanced the contrast in the vascular
structures such as hepatic vein and inferior vena cava. The paramagnetic NO-Fe-DTC metal
complex is also a potential MRI signal enhancer and acts as contrast agent. These contrast
agents showed the magnetic relaxation changes of neighboring protons to visualize the NO
generated in living animal tumors [Jordan et al. 2000]. Other contrast enhancement effect
showed an impact of short NO exposure to hemoglobin during MRI signal recording as
source of in vitro MRI and in vivo functional MRI (fMRI) [Di Salle, et al. 1997]. fMRI signal
intensity of venous blood in T1-, T2-, and T2*-weighted images proportionately changed
with NO real-time generation in brain. Later, different approaches of blood hemoglobin and
NO interaction were attempted to monitor fMRI signal sensitive to NO: mainly metHb and
NO-Hb enhanced the MRI signal intensity. These observations suggested a blood flowindependent effect and less utility [Di Salle, et al. 1997]. Still it is hope that NO sensitive
fMRI techniques can detect slow epithelial intracellular processes such as metabolic
integrity, vascular tonicity, stress, shear and inflammatory effects at early stages of the
disease processs, allowing precise monitoring of onset in intact biological systems at cellular
level. Other approaches are also emerging to use NO biosensors for multimodal
imaging. Currently, use of fMRI as a non-invasive NO sensitive technique has emerged as
potential and remarkable tool to detect apoptosis in vivo. The NO sensitive fMRI techniques
can detect slow epithelial intracellular processes such as metabolic integrity, vascular
tonicity, stress, shear and inflammatory effects at early stages of the process, allowing the
onset in intact biological systems, providing a useful tool for monitoring at cellular level.
2.2 The source of intracellular NO and metabolic integrity- feasibility of MRI
The nitric oxide is released from the L-arginine in tissue along with molecular oxygen in the
oxidative L-arginine degradation reaction of L-arginine pathway catalyzed by either of any
three different NO synthase (NOS) isoenzymes. NO controls the intracellular redox state in
tissue and protects the metabolic integrity in two ways [Kuppusamy, et al. 1994]. First,
anions and cations in intracellular space or cellular redox state prevent apoptosis for
example, NO in hepatocytes, neurons, glial cells and fibroblasts controls the release of
mitochondrial apoptogenic factors and induces apoptosis by activation of caspases
[Hortelano, et al.2005 ]. Second, peroxynitrites accumulate as product of nitric oxide and
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Enzyme Inhibition and Bioapplications
superoxide anion. Peroxynitrites promote apoptosis through the indirect activation of
caspases [Kuppusamy, et al. 1994, Hortelano, et al.2005]. However, all these approaches are
still invasive to evaluate apoptosis.
Overproduction of nitric oxide and imaging the NO accumulation due to neurotoxicity was
reported in neurological disorders or neurodegeneration. The reason of neurotoxicity was
reported due to NO reactive oxidative properties in tissue [Caramia, et al. 1998]. This
approach was reported not to solve the purpose of in vivo functional monitoring but NO
properties indicated the state of tissue and follow-up of dynamic status of
neurodegenerative factors in brain [Caramia, et al. 1998]. In other application of NO
bioimaging to detect apoptosis, T2 weighted maps from control vs GSNO treated and
z.VAD treated animals exhibited hyperintense areas perhaps due to toxicity of NO on the T2
maps while z-VAD treated animals showed small lesion areas due to reduced NO toxicity.
The response of z-VAD injection was presumed as reduced toxicity due to caspases and NOdependent apoptosis. These authors explained that GSNO increased T2 intensity 25% while
z-VAD reduced this MRI signal [Komarov, et al. 1995]. These classic reports indicated that
T2 hyperintensities on MRI positively offer the possibility of in vivo evaluation of cell death
at different locations in whole tissues undergoing apoptosis. The apoptosis is also a major
mechanism in brain neurodegeneration and post-myocardial infarction heart [Caramia, et al.
1998]. However, this approach seems as potential tool of functional imaging in near future
to monitor the therapeutic intervention of new drugs to reduce neurodegeneration[
Kuppusamy, et al. 1994, Hortelano, et al.2005, Foster, et al. 1998]. Currently, non-invasive
NO sensitive techniques are big hope as potential and remarkable tools to detect apoptosis
in vivo.
Initial studies on NO with iron-dithiocarbamate complexes had succeeded in direct
detection of NO in mice by whole body electron paramagnetic resonance spectroscopy
(EPR) at the L-band [Fujii, et al. 1997, Komarov, et al. 1995] with new possibilities by
Magnetic Resonance Spectroscopy [Reif, et al. 2009, Li, et al. 2009]. Several authors
demonstrated the feasibility of EPR imaging in visualizing free radical distributions in vivo
at low resolution where the intrinsic line width of the radical is large, such as the spintrapped NO [Yoshimura, et al. 1996, Kubrina, et al. 1992, Foster, et al. 1998]. This drawback
of low resolution caught attention for feasible MRI contrast agents, such as stable (N-methylD-glucamine)2-Fe(II)-NO complex to generate better resolution. The complex has a much
longer in vivo half-life than most (stable) nitric oxide derived compounds. Recently Nmethyl glucamine iron complexes have shown greater affinity with NO to make (MGD)2Fe(II)-NO complex useful for in vivo NO measurements by EPR [Caramia, et al. 1998]. In
recent years, the art of MRI combined with NO spin trapping mechanism was evaluated as
feasible method of mapping the distribution of NO spin-trap complex in animals with
possibility in clinical use. In following section, our design of ultrafast MRI protocol is
described using NO/NOS sensitive biosensor complexes for use at 21-Tesla MRI
microimager.
2.3 Approach of stable NO sensitive lipopolysaccharides (LPS) as feasible imaging
complexes
The LPS serves as encaged bag holding contrast agent. The LPS based NO imaging
approach has following presumptions:
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



127
spin-trapped NO is stable in intracellular tissue environment;
NO-LPS contrast enhancement properties are MRI visible;
NO-LPS complex is stable in tissues and organs; and
simultaneous visualization and mapping of NO free radicals are possible by MRI.
The routine method of LPS injected animals after 6 hours of (MGD)2-Fe(II) injection, usually
serve as experimental model of subsequent MRI detection and visualization of NO
generated in animals. It was confirmed by using a specific NOS inhibitor N-monomethyl Larginine (L-NMMA) [Fujii, et al. 2007]. The observations indicated that the suppressed NO
levels were proportional to NO sensitive LPS concentration independent of (MGD)2-Fe(II)NO contrast agent [Fujii, et al. 2007]. Next issue of feasibility and stability of (MGD)2-Fe(II)NO imaging contrast agent was major breakthrough reported in model systems to generate
EPR and MRI signals. It is established that the NO complex (MGD)2-Fe(II)-NO is very stable
in model aqueous media. The NO complex (MGD)2-Fe(II)-NO complex, if injected, is stable
in tissues and organs as confirmed by L-band EPR measurements as reported elsewhere
[Lai, et al. 1994, Mulsch, et al. 1999]. These authors confirmed the assignment as (MGD)2Fe(II)-NO from the hyperfine coupling J constant of this signal (see Figure 5). However, LPS
complex accumulates in liver, brain, heart, kidney [Yoshimura, et al. 1996]. NOS inhibitor,
N-monomethyl L-arginine (L-NMMA, 2 mM) confirmed that the NO complex is not bioreduced or biodegraded in vivo based on the EPR spectrum of (MGD)2-Fe(II)-NO complex.
The stability of (MGD)2-Fe(II)-NO complex was at least 12 hours. In other report, the
reduction/ decomposition of the NO complex occurred in the presence of 1 mM ascorbic
acid or glutathione inhibitor with a half-life of 40 and 48 min, respectively [Perrier, et al.
2009]. All these evidences indicate the NO complex as stable, non-biodegradable or
decomposed form of (MGD)2-Fe(II)-NO complex as strong proton relaxation enhancer with
paramagnetic properties [Perrier, et al. 2009, Fujii, et al. 2007]. All these evidences also
indicated the NO complex as stable, non-biodegradable or decomposed form of (MGD)2Fe(II)-NO complex as strong proton relaxation enhancer with paramagnetic properties.
Biosensor#
DNIC (240 µmol/L)
DNIC-cysteine
DNIC-GSH(230 µM)
DNIC-BSA(160 µM)
MNIC-MGD
MGD
(MGD)2-Fe(II)-NO T1
(100 nmol/g)
T2
(MGD)2-Fe(II)
T1 (600/15)
msec
931 ±3 7
850 ± 24
247 ± 13
55.5
13.3*
8.3*
24.9
r1
L.mmol-1.sec-1
r2
L.mmol-1.sec-1
0.23 ± 0.06
0.11 ± 0.02
0.71 ± 0.09
0.97 ± 0.09
0.33 ± 0.03
0.19 ± 0.03
1.37 ± 0.30
1.37 ± 0.03
52.4
13.9**
8.7**
25
# The strong magnetic moment of the unpaired electron promotes both spin lattice and spin-spin
relaxation of the surrounding water protons, resulting with decrease in their spin-lattice (T1) and spinspin (T2) relaxation times or enhancement in signal intensity on T1 or T2 weighted MR images.
Relaxation constants of (MGD)2-Fe(II)-NO, (MGD)2-Fe(II) at 500 MHz* and 300 MHz** is shown
indicating that relaxivity is not magnetic field dependent over this range.
Table 2. Relaxation constants of biosensor complexes are shown for NO detection in
aqueous solutions.
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Enzyme Inhibition and Bioapplications
In other study, NO complex was prepared from NO gas and (MGD)2-Fe(II) in glass
capillaries. The strong magnetic moment of the unpaired electrons promote both spin lattice
and spin-spin relaxation of the surrounding water protons and show a decrease in their
spin-lattice (T1) and spin-spin (T2) relaxation constants or show enhanced signal intensity in
T1 or T2 weighted MR images as shown in Table 2 and Figure 5 [Hortelano, et al. 2005].
Major points were:




The relaxation constants of (MGD)2-Fe(II)-NO in aqueous media were characteristic and
complex concentration dependent;
T1 and T2 relaxation constants of liver were measured 1.17(1/mM * sec) and 1.32
(1/mM * sec) respectively at 500 MHz at increasing concentration of NO;
T1 relaxation constant of (MGD)2-Fe(II), was reported 0.044 (1/mM * sec) at both 20
MHz and 85 MHz magnetic fields. The T1 relaxation constant was field independent;
Distinct increase in relaxivity after complexing NO with (MGD)2-Fe(II). These unique
properties of complex suggested its feasibility to visualize the regional distribution in
tissue in vivo where the NO was trapped.
In previous study, (MGD)2-Fe(II)-NO and LPS complexes were used as in vivo MR contrast
agents [Fujii, et al. 2007]. First, intraperitoneal injection of 2 ml of 9.1 mM (MGD)2-Fe(II)-NO
complex (150 nmol/g tissue) showed relative difference on T1-weighted MR images of rat
liver site before and after injection of NO complex as shown in Figure 5.

The images indicated the NO complex very effective ‘‘intrinsic contrast agent,’’ to
enhance the contrast in several other organs.
The Second, LPS injected animals after 6 hours of (MGD)2-Fe(II) injection, showed
subsequent MRI detection and reproducible visualization of NO generated in liver of
animals as shown in Figure 5 and Fig. 6a (indicated by the arrow) as plots against time with
the intensity of the reference water sample. Figure 6b showed that the image intensity in the
liver increased with the NO generation, without any intensity change in reference sample in
the presence of the specific NOS inhibitor N-monomethyl L-arginine (L-NMMA).
Interestingly, other face of (MGD)2-Fe(II)-NO complex is ‘spin trap’. Spin traps are chemical
compounds used to detect unstable free radicals such as hydroxyl (*OH) and superoxide (O2*-)
in biological systems. Spin trapping compounds (spin traps) selectively react with NO and
trap the nitric oxide. The spin-trap stock (MGD)2-Fe(II) complex solution (mixture of
deoxygenated saline MGD (100 mM) and FeSO4 (20 mM) solutions in nitrogen) makes
(MGD)2-Fe(II)-NO complex by mixing (MGD)2-Fe(II) with either pure NO gas or S-nitroso-Nacetyl DL-penicillamine (SNAP). The pure NO gas is prepared by passing NO gas through a
KOH solution. Major concern was the coordination of NO to the Fe complex altered the
solubility of N0-Fe(II)(MGD)2 < 1 mM. The in vivo half life of N0-Fe(II)(MGD)2 was 41 minutes
and only 40% trapped NO was estimated by detection of Fe(II)(MGD)2 induced inhibited NOS
and decrease in NO formation and perfusion in the tissue [Fichtlscherer, et al. 2000].
2NO + Fe(III)(MGD)2
H2O (*OH)_______ NO-Fe(II)(MGD)2 + NO2-
NO + Fe(III)(MGD)2 ____Reducing Equivalent____ NO-Fe(II)(MGD)2 (40%)
(1)
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129
MR contrast due to trapped NO can be expressed in Eq 2, 3:
1/T1[NO-Fe(II)(MGD)2] = 1/T1(observed) – 1/T* = r1i[NO-Fe(II)(MGD)2] × concentration (2)
1/Tobs = 1/T*+ r1i[NO-Fe(II)(MGD)2]; i = 1, 2...
(3)
where T1(observed) is long longitudinal relaxation time constant and T1* is short longitudinal
relaxation time constant in absence of [NO-Fe(II)(MGD)2], r1i is longitudinal relaxivity
dpends on I = 1, 2, 3 the number of carbons in branch of MGD chain, [NO-Fe(II)(MGD)2],
concentration and interaction with NO specific bioactive receptor, enzyme, antibody,
hormone molecules in tissue.
Fig. 5. Imaging of preformed (MGD)2-Fe(II)-NO complex. Transverse T1-weighted MR
images in the axial plane of the liver of Wistar rats. Two ml of 9.5 mM (MGD)2-Fe(II)-NO
complex was injected i.p. in the lower abdomen of 250 g rats (n = 3). a: Control, before
injection; b: 60 min after injection of (MGD)2-Fe(II)-NO complex. A sketch at bottom shows
the interaction of No with heme as basis of MRI signal. Reproduced with permission for
reference Fujii, et al. 2007.
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Enzyme Inhibition and Bioapplications
Fig. 6. Imaging of NO in six phantoms and LPS-treated rats. a: Transverse T1-weighted MR
images focussed on a selected region of the liver in LPS-doped rats. The MR images were
measured at the times indicated. Six hours after LPS injection, the NO spin-trap [3 ml of
(MGD)2-Fe(II), MGD: 100 mM, Fe: 20 mM] was administered i.p. b: Plot of MR image intensity
with time. Signal intensities were averaged over the selected region indicated by the arrow in a
(7*7 mm2) for three different animals (filled symbols), normalized to intensity of a reference
(water in a tube placed next to the animal, open symbols). At zero time, the spin-trap, (MGD)2Fe(II), was added. c: Comparison of the signal intensity in a selected slice region of the liver in
the presence and absence of the NOS inhibitor, L-NMMA. The image intensity in the liver was
first normalized to the external standard noted above. The averaged intensity in the selected
regions (7 x 7 mm2) was averaged (n = 3). L-NMMA (50 mg/kg in saline) was injected i.p. 3 h
after LPS injection (Reproduced from reference Fujii et al. 2007).
2.4 MRI measurement protocol design
We developed a MRI protocol to investigate the regional distribution of (MGD)2-Fe(II)-NO
in various organs. For whole animal imaging, protocols for relaxivity of (MGD)2-Fe(II)-NO
were designed on a Bruker 500 MHz, to achieve millimolar relaxivity. For the animal
Inhibition of Nitric Oxide Synthase Gene Expression:
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131
experiments, anesthesia was administered as sodium pentobarbital (30 mg/kg, i.p.) and
ketamine (20 mg/kg, intra-muscular injection) before injection of contrast agent in animals.
For imaging, animals were anesthetized by intubating with 14 gauge, 2 inch intravenous
catheter (Abbocath Lab, IL) on nose with 30% oxygen/70% nitrogen mixture containing 5%
isoflurane/air mixture to continuous supply through nose during MRI session. Animals
were kept in vertical direction to the side of MRI gantry and 30 mm diameter RF insert
covering kidneys in the center of magnet. During six hours after administration of LPS to
Wistar rats (150–200 g) they were injected intraparitoneally with 3 ml of (MGD)2-Fe(II)
[MGD:100 mM, Fe:20 mM] at different intervals. MR images of the anesthetized rats (n = 5)
were obtained after 1 hr spin trap injection with a Bruker Biospin 500 MHz scanner
(PARAVISION 3.2).
Different imaging techniques are available in our lab on 500 MHz and 900 MHz scanners. In
vivo NO induction was carried out according to previously reported methods with slight
modifications [Lai, et al. 1994, Yoshimura, et al. 1996, Green, et al. 1998].




Axial T2-weighted spin echo sequence with fat suppression (TR 2000 ms, TE 100 ms,
flip angle 30; EC 1/1 15.6 kHz) was used for more detailed T2-weighted information in
detecting solid tissues.
Axial T1-weighted images were acquired at TR/TE/flip angle = 750 ms/ 4.18 ms/25º,
FOV/matrix size/spatial resolution = 2.6 x 1.4 cm2/ 256 x 256 ×75 μm, and the
inversion time (TI approximately 250 ms) set to null the normal the background tissue.
Gradient echo sequence in-phase and opposed-phase(TR 180 ms, TE 2.3 ms/4.5 ms, flip
angle 30 were done as a dual echo sequence to show renal lesions hypointense and
parenchyma isointense with hyperintense protein rich cyst. Opposed phased T1weighted gradient echo sequences were sensitive to fat.
Axial T1-weighted gradient echo sequence for dynamic perfusion/ diffusion weighted
imaging (TR 130 ms, TE 1.0 ms, flip angle 30) was done after 30 ml intravenous
gadolinium contrast injection for acquiring pre-contrast and post-contrast images in
arterial phase to distinguish perfusion in the tissues. Parameters for T1-weighted spinecho images were TR 1500 msec, TE 10 msec, 2 NEX, 1-mm slice thickness, 0.5-mm slice
gap, field of view 12 x 3 x 12 cm3, and matrix, 256 x 256.
The original bird-cage coil with 2.6 cm diameter Rf insert was used for MR imaging. We
established in lab MR image analysis by shareware software NIH Image J, a PC version
supplied by Scion Corporation. The analysis of averaged image intensities from selected
regions of interest 15 x 3 x 15, and matrix , 256 x 256 served as best for measurement of
signal enhancement. An external reference (saline solution in a sealed test tube) placed near
the animal serves to normalize intensities from both control and test groups. Percentage
enhancements can be calculated from the normalized data [35]. Functional dependence of
the MR imaging signal intensity S on the intrinsic properties (proton spin density N(H), T1,
T2) has been derived based on the assumption of a monoexponential dependence on T1 and
T2 with a spin-echo sequence: S(TE,TR) = N(H)exp(-TE/T2) [1 -exp[-(TR/ TE)/T1]]. The
term on the right reflects the T1 dependence, and the exponential expression on the left
reflects the T2 effects. TE << T2 generates a T1-weighted. T1 image is obtained by means of
least-squares fitting (Marquardt-Levenberg algorithm) of the mean intensity values versus
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Enzyme Inhibition and Bioapplications
TR. T2 weighted image is obtained by TR >> T2. Mean intensity values are fitted versus the
TE (Marquardt Levenberg algorithm).
T1 weitghted MR contrast due to trapped NO as NO-Fe(II)X spin trap in tissue can be
expressed in Eq 4, 5:
1/T1[NO-Fe(II)X] = 1/T1(observed) – 1/T* = r1i[NO-Fe(II)X] × concentration
(4)
1/Tobs = 1/T*+ r1i[NO-Fe(II)X]; i = 1, 2...
(5)
where T1(observed) is long longitudinal relaxation time constant and T1* is short longitudinal
relaxation time constant in absence of [NO-Fe(II)X],
r1i is longitudinal relaxivity dpends on i= 1, 2, 3 the number of carbons in branch of X
contrast agent, its concentration and interaction with NO specific bioactive receptor,
enzyme, antibody, hormone molecules in tissue.
2.5 Approaches to NO evaluation by Magnetic Resonance Imaging (MRI) techniques
Two major developments reported in context with NO. First, a search of NO specific stable
multimodal MR/EPR sensitive spin trap complex; second, calibrating imaging
characteristics of spin trap complex on NMR/MRI/PEDRI techniques. NO is perfused in
gaseous phase or remains in ionic phase by redox state in the tissues. Both forms of NO are
captured or trapped by Fe(II)-(DTC)2 complexes and show MRI signal change due to
reactive nitrosylation (reaction of intracellular NO with Fe(II)(DTC)2 to form NOFe(II)(DTC)2 complex) shown in Eq 6. Second-order rate constant of the reaction of NO with
Fe(II)(ProDTC)2 was reported to be (1.1  0.3)×108 M-1 s-1[Caramia, et al. 1998].
Fe(II)(DTC)2Fe(II)(ProDTC)2 + NO  NO-Fe(II)(DTC)2
(6)
Recently, attempts have been made to overcome the limitations of EPR-based imaging with
low resolution and large sample size by employing new approaches of nuclear magnetic
resonance (NMR) and magnetic resonance imaging (MRI) techniques. Several paramagnetic
metal complexes have been widely used as contrast agents in magnetic resonance imaging
(MRI) due to their ability to enhance the NMR relaxation of neighboring protons [von
Bohlen, et al. 2003]. Paramagnetic NO-Fe-DTC complex has been utilized to visualize NO
generated in living rats with septic shock [Fichtlscherer, et al. 2000, Fujii, et al. 1999].
Paramagnetic nitrosyl iron complex showed contrast properties to enhance the signal
intensity of SNP-perfused rat liver in MRI [Fichtlscherer, et al. 2000, Fujii, et al. 1999]. Thus,
a paramagnetic nitrosyl iron complex may be potentially useful as a microscopic localizer
MRI contrast agent specific for NO accumulation in living organisms. Other protonelectron-double-resonance-imaging (PEDRI) technique was based on the enhancement of
proton NMR signal intensity in the presence of radicals through the Overhauser effect
Mulsch et al. 1999,Foster, et al.1998]. Rat livers were exposed to SNP as an NO donor
exhibited intense PEDRI images [Rast, et al. 2001, Jordan, et al.2004]. PEDRI imaging is a
potential tool to study NO in large biological specimens. Still imaging application of MR
and EPR spectroscopy using nitric oxide synthase inhibitors, oxygenation or perfusion are in
infancy [von Bohlen, et al. 2003, Zhou, et al. 2009, Nagano, et al. 2002, Anbar, 2000]
Inhibition of Nitric Oxide Synthase Gene Expression:
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133
NO-Fe(II)(DTC)2
Fig. 7. NO sensitive biomarker imaging contrast agents. Example of NO-Fe(II)(DTC)2 is cited
to illustrate the signal change during changes in cerebral blood flow and possibility of
functional MRI [Caramia, et al. 1998].
Two decades ago, NO activation-induced signal changes in functional MRI (fMRI) was
observed [Di Salle, et al. 1997]. Hb–NO was found to produce marked stimulator dose and
concentration-dependent NO sensitive fMRI signal intensity changes (increased after
aqueous NO solution, nitrite, or dithionite and nitrite added to blood while it decreased on
the addition of ascorbic acid) examined by T1-, T2-, and T2 -weighted MRI in venous
erythrocytes with possibility of physiological MRI evaluation associated with NO change
[Di Salle, et al. 1997]. Transient formation of two paramagnetic species (met Hb and NO-Hb)
enhanced the signal intensity, while ascorbate reduced the signal intensity in fMRI
experiment on healthy volunteers during standard tasks. Further, infusion of NO precursor,
L-arginine increased the cerebral blood volume (measured by MRI) in nonischemic
spontaneously hypertensive rats [Foster, et al. 1998]. In other study, administration of an
NO donor, isosorbide dinitrate (ISDN) increased both tumor blood flow and partial oxygen
pressure in mice implanted with liver tumor in the thigh [Jordan, et al. 2000]. These findings
indicate blood flow-independent change in MRI signal produced by different additives or
stimulators. These observations open a new perspective on the monitoring of NO and the in
vivo speculation of NO effects to test new drugs by using magnetic resonance techniques.
Fuji et al. 1999 first time reported in vivo MRI of NO using Fe(MGD)2 as NO–Fe(MGD)2 spintrapping technique in a septic shock rat model [Fujii, et al. 1999]. (Fig. 6A). The reduced
signal after administration of an iNOS inhibitor confirmed the MRI visibility of NO–
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Enzyme Inhibition and Bioapplications
Fe(MGD)2 complex especially in inflammation which has high levels of NO. Now many
MRI visible nitrosyl-iron complexes are known for MRI mapping of NO using a
conventional 1.5 T MR scanner. The principle is that after exposure to NO in tissues
Fe(MGD)2 imaging agents form nitrosyl-iron complexes, which shorten the T1 and T2
relaxation time in a concentration-dependent manner [Hong, et al. 2009]. Mainly, four major
techniques are reported in quest of developing new NO sensitive MRI contrast agents. First,
unpaired electrons of the nitrosyl–iron complexes can enter into dynamic nuclear
polarization with water protons, a technique called proton-electron-double-resonance
imaging (PEDRI). Second, superparamagnetic iron oxide (SPIO) or ferumoxides dephase T2weighted MRI signal dependent on SPIO: NO donor ratios indicating inverse relation of
contrast enhancement by SPIO with increasing levels of NO in tissue. The MRI signal
change was due to reduction of ferric to ferrous iron with result of decrease in paramagnetic
relaxation of water protons [Kojima, et al.2001]. Third, technical development emerged as
irreversible paramagnetic chemical exchange saturation transfer (PARACEST) based MRI
contrast agents in imaging [Liu, et al. 2007, Zhang, et al. 2003, Wu, et al. 2008]. A
PARACEST MRI agent, Yb(III)-(1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid)orthoaminoanilide (Yb-DO3A-OAA), was developed for NO detection (Fig. 6B) [9]. Fourth,
NO and O2 combine by irreversible covalent bond that causes a quick disappearance of the
PARACEST effect captured in the MR images. PARACEST MRI is fast data acquisition (in
few minutes) vs SPIO based imaging is slow (in few hours). PARACEST effect has poor
sensitivity (only detects millimolar concentrations of NO).
2.6 Approach of real-time NO synthase inhibition as blood oxygenation dependent
MRI signal
Nitric oxide synthase inhibition (NOSi) in humans by blood oxygenation level-dependent
(BOLD) MRI as reduced response to NOSi was reported using BOLD MRI to compare
changes in R2* to direct measures of renal medullary oxygen levels and blood flow using Nnitro-L-arginine methyl ester invasive probes (OxyLite/OxyFlo) to examine for the first time
the effect of NOSi on intrarenal oxygenation in humans [Li, et al. 2009, Di Salle, et al. 1997].
Authors showed that NOSi decreased medullary pO2 and blood flow in a dose-dependent
manner, and BOLD MRI showed an increase in medullary R2* consistent with the invasive
pO2 measurements.
Other alternate approach of positron emission tomography (PET) was proposed in recent
study using nitric oxide imaging probe prepared from [18F] 6-(2-fluoropropyl)-4-methylpyridin-2-amine by substitution at position-6 of substituted 2-amino-4-methylpyridine with
favorable properties as a PET tracer to image iNOS activation/expression with PET of
animal lungs [Zhou, et al. 2009].
3. Major developments in NO bioimaging probes
3.1 Magnetic resonance relaxation enhancement mechanism
In 1999, the first in vivo MRI of NO was reported using the spintrapping technique in a
septic shock rat model [Fujii, et al. 1999]. NO was trapped by Fe(MGD)2 in vivo, which can
be visualized by MRI. The NO–Fe(MGD)2 complex displayed significantly enhanced
Inhibition of Nitric Oxide Synthase Gene Expression:
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135
contrast in the vascular structure, such as the hepatic vein and inferior vena cava, which can
be reduced after administration of an iNOS inhibitor.Initially, NO mapping was reported by
whole body EPR imaging at poor resolution [Claudette, et al. 2005,Yoshimura, et al. 1996].
Recently, MRI strategies combined with in vivo spin-trapping map ‘‘NO distributions’’
within tissues and organs showed much higher spatial resolution using (MGD)2-Fe(II)-NO
as a NMR contrast agent in vivo [Fichtlscherer, et al. 2000]. The MR images also provided
follow-up of NO generation kinetics at different sites within the organ at much higher
spatial resolution than with EPR [Claudette, et al. 2005,Yoshimura, et al. 1996]. NO makes
stable complex such as (MGD)2-Fe(II)-NO. In vivo, hemoglobin is normally the natural NO
spin-trap such that NO tends to bind with hemoglobin or oxidize the hemoglobin, followed
by conversion to nitrosyl-hemoglobin or methemoglobin, both of these complexes are
paramagnetic. Authors believe that during brain stimulation, NO is generated in some
regions and it is quite possible that (paramagnetic) nitrosyl-hemoglobin and methemoglobin
are formed in these active regions in brain. The MRI signal intensity enhancement in these
regions results from changes in blood flow rich with these complexes. However, for blood
flow–independent effects in MRI, the paramagnetic relaxation from spin-trapped NO might
provide a new fMRI contribution in future. To our knowledge, there have been no reports
successfully imaging or detecting free radicals such as NO in vivo by NMR or fMRI.
Further, both short lifetime and rapid diffusion of NO in time preclude any effective
relaxation enhancement mechanisms. NMR spin trapping serves to overcome these
problems such as NO short lifetime and its fast diffusion in tissues to some extent. In other
classic study on rats, spin relaxation constants of dinitrosyl-iron bound albumin or GSH and
mononitrosyl-iron dithiocarbamate MGD complexes showed concentration dependence by
MRI technique [Fujii, et al.1999].
3.2 Emission enhancement mechanism
Present state of art in NO detection was developed over last 2 decades using fluorescein
cheletropic compounds visualized by emission enhancement and photoinduced electron
transfer mechanisms. Fluorescence properties of fluorescein derivatives are controlled by
the photoinduced electron transfer (PET) process from the benzoic acid moiety to the
fluorophore, the xanthene ring. PET is a widely accepted mechanism for fluorescence
quenching in which electron transfer from the PET donor to the excited fluorophore
diminishes the fluorescence of the fluorophore. A brief survey of these compounds and their
fluorescence mechanisms will catch the attention of readers with the newer developments of
NO detection in tissues. EPR (also known as electron spin resonance or ESR) explores the
magnetic moment associated with the unpaired electron(s) in free radicals and paramagnetic
metal ions [Mader, 1998, Swartz, 2007]. It has been applied for the detection of free radicals
and paramagnetic metal ions in various biological conditions [Berliner, et al. 2004]. When an
unpaired electron in a paramagnetic molecule is subjected to an external magnetic field, the
energy level of the electron splits into two quantum states with different energy levels.
Typically, the magnetic field is scanned with a fixed microwave frequency of approximately
9.5 GHz (usually called the X-band). Specimens less than about 100 μL are measurable in an
Xband spectrometer. However, larger biological samples (e.g., tissues, organs, and live
animals) cannot be measured with a conventional X-band spectrometer due to the high
dielectric loss of water at such frequencies and the small size of the EPR cavity resonator.
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Enzyme Inhibition and Bioapplications
EPR spectroscopy at lower frequencies, the L-band (0.4–1.6 GHz) or S-band(1.6–4 GHz), can
be used for in vivo EPR imaging. EPR imaging is generally considered to be the most
effective technique for noninvasive observation of the spatial distribution of free radicals
[Kleschyov, et al. 2007]. Direct EPR detection of endogenous free radicals in biological
samples requires that the radical of interest is present at a concentration higher than the
practical detection limit (0.1–0.01 μM) and has a relatively long lifetime. Although such
requirement can be met at cryogenic temperature, in vivo detection is almost impossible
since most radicals do not have a sufficiently long lifetime. Therefore, spin-trapping
techniques have been developed in which the spin traps can react with unstable free radicals
to form a relatively stable adduct [Berliner, et al. 2004, Davie, et al. 2004]. The formation of
the long lived adduct results in accumulation of steady-state free radicals which can be
detected readily by EPR spectroscopy.
In principle, the fluorescence properties of fluorescein derivatives are controlled by the
photoinduced electron transfer (PET) process from the benzoic acid moiety to the
fluorophore, the xanthene ring. PET is a widely accepted alternate mechanism for
fluorescence quenching in which electron transfer from the PET donor to the excited
fluorophore diminishes the fluorescence of the fluorophore. The fluorescein structure has
two parts confirmed by X-ray analysis, i.e., the benzoic acid moiety as the PET donor and
the xanthene ring shown as the fluorophore in previous study [Liu, et al. 2007]. The study
showed that only small alterations in absorbance were observed among fluorescein and its
derivatives. The dihedral angle () between the benzoic acid moiety and the xanthene ring
was almost 90o. It also suggested that there is little ground-state interaction measured by
HOMO energy levels between these two parts.
The relationship of HOMO energy levels and () angle of fluorescent compounds predict the
fluorescence mechanism of benzoic acid moiety in the compounds. If the HOMO energy
level of the benzoic acid moiety is high enough for electron transfer to the excited xanthene
ring, the  value will be small. In other words, fluorescein derivatives with a high  value
must have benzoic moieties with low HOMO energy levels. The HOMO energy levels of 3aminobenzoic acid, 3-benzamidobenzoic acid, 3,4-diaminobenzenecarboxylic acid (DABCOOH), and benzotriazole-5-carboxylic acid (BT-COOH), which are the benzoic moieties of
aminofluorescein, benzamidofluorescein, DAF-2, and DAF-2T, respectively were estimated
by semi-empirical (PM3) calculations. DAB-COOH and aminobenzoic acid, which are the
benzoic acid moieties of weakly fluorescent fluorescein derivatives, have higher HOMO
levels than BT-COOH and amidobenzoic acid. These results were consistent with the PET
mechanism.
Further, to confirm this mechanism 9-[2-(3-carboxy)-naphthyl]-6-hydroxy-3H-xanthen-3-one
(NX) and 9-[2-(3-carboxy)anthryl]-6-hydroxy-3H-xanthen-3-one (AX) were synthesized
[Nagano, et al. 2002]. It is well-known that expanding the size of aromatic conjugates makes
their HOMO levels higher, similar with by introducing electron-donating substituents.
Thus, naphthoic acid and anthracene-2-carboxylic acid would have higher HOMO levels
than benzoic acid, and therefore, the fluorescence properties of NX and AX would differ
from those of fluorescein.. The absorbance maximum (Exmax) and emission maximum
(Emmax) of NX and AX were closer and not much altered among these fluorescein
derivatives. However, the  values were greatly altered; NX is highly fluorescent, whereas
Inhibition of Nitric Oxide Synthase Gene Expression:
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137
AX is almost nonfluorescent [Nagano, et al. 2002]. Thus, a small change in the size of
conjugated aromatics, namely, from naphthalene to anthracene, causes a great alteration of
the fluorescence properties. Authors compared the HOMO levels of the benzoic acid
moieties with xanthine ring and naphthoic acid. Napthoic acid moiety is present in highly
fluorescent fluorescein and NX. The HOMO levels of benzoic acid moiety were lower than
that of the xanthene ring, while the HOMO level of anthracenecarboxylic acid moiety in
fluorescent AX, was higher than that of the xanthene. These results were consistent with the
idea that a PET process controls the fluorescence properties of fluorescein derivatives and
that these properties can be predicted from the HOMO level of the benzoic acid moiety,
with a threshold at around -8.9 eV. This in turn provides a basis for developing novel
fluorescence probes with fluorescein-derived structure. Still much remains to justify the
fluorescent probes in NO bioimaging.
Fig. 8. Representative spin-trapping agents for EPR imaging of NO.
Fe(MGD)2 or Fe(DETC)2 have also been tested for EPR imaging to map the spatial
distribution of NO generation in the ischemic myocardium [Kuppusamy, et al. 1996].
Subsequently, Fe(MGD)2 was used in both EPR and MR imaging in rats with sepsis shock
[Fujii, et al.1999]. It was reported that Fe(MGD)2 can reduce nitrite to NO at physiological
pH, which may cause some detection Inaccuracy as shown in Figure 8, although the rate
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Enzyme Inhibition and Bioapplications
constant is quite slow (1–5 M−2 s−1) [Tsuchiya, et al. 2000]. Fe(DETC)2 has been used for
imaging vascular NO production in rabbits [Kleschyov, et al. 2000], animal brain injury
[Ziaja, et al. 2007], vascular disease [Khoo, et al. 2004], and even in plants [Xu, et al. 2004,
Vanin, et al. 2007]. Heme proteins, such as myoglobin, cytochrome c, and catalase, have
been used as NO-trapping agents in various biological specimens[Kleschyov, et al. 2000].
Although the EPR signals of heme–NO species can be observed in tissues exposed to
elevated levels of NO, it is almost impossible to discriminate between the individual
nitrosylated heme proteins due to overlapping of the signal. Generally, the dominant heme–
NO EPR signal belongs to the major heme protein expressed in the tissue (e.g., myoglobin in
the heart [Konorev, et al. 1996]). The most frequently used heme protein in EPR research is
hemoglobin (Hb) [Kosaka, et al. 1994], which can interact with NO to generate several
paramagnetic Hb–NO derivatives detectable by EPR spectroscopy [Picknova, et al. 2005].
Different preparations of deoxy-Hb have been used for NO trapping in isolated cells and
organelles [Kozlov, et al. 1996].
3.3 NO imaging and fluorescent dyes
Several fluorescence indicators for direct detection of NO have been generated, which might
be useful for in vitro or in vivo NO-imaging. These substances interact with NO to form a
fluorescent complex. Several fluorescent dyes have been developed, which can be used for
NO bioimaging. However, several of these promising fluorescent dyes have been found not
to be very specific such as 2′,7′-dichlorofluorescein (DCF) or to be cytotoxic such as 2,3diaminonaphthalene (DAN). Of specific mention, iron (II) N-(dithiocarboxyl)
sarcosine(Fe(DTCS)2; Fig. 5) trapping agents within 1 h after injection of the spin trap
generated stable signal which indicated that the formation of the steady-state free radical
had reached equilibrium. The same trapping agent was also tested for NO imaging in
lipopolysaccharide-treated mice [Quaresima, et al. 1996]. Subsequently, EPR measurements
and EPR-CT (CT denotes computed tomography) imaging with Fe(DTCS)2 were carried out
in live mice [Yokoyama, et al. 1997]. The use of Fe(DTCS)2 resulted in a clear EPR-CT image
showing high intensity areas in the ventral regions, while other NO-bound iron complexes
such as Nmethyl-D-glucamine dithiocarbamate (Fe (MGD)2; Fig. 5) or N,Ndiethyldithiocarbamate (Fe(DETC)2) gave much lower signal-to-noise ratios. Several compounds
such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO; Fig. 5), α-phenyl-N-tertbutylnitrone(PBN), or 3,5-dibromo-4-nitrosobenzene sulfonate (DBNBS), NO cheletopic
traps (NOCTs), aci anions of nitroalkanes, iron (II)–dithiocarbamate(DTC) complex
and(DAFs) are fluorescein derivatives. DAFs themselves scarcely fluoresce while DAF-Ts
are strong fluorescent compounds. However, the mechanisms accounting for the diminution
of fluorescence in DAFs and fluorescence enhancement in DAF-Ts remain unclear.Other
fluorescent Fe(DTCS)2 and Cobalt based dyes have been tested in some cell culture systems,
but these substances have not been tested in in vivo or in vitro acute brain slice preparations
or tissues [von Bohlen, et al. 2003, Zhou, et al. 2009]. Until now, several other fluorescent
dyes have been promising candidates for the evaluation of the temporal and spatial aspects
of NO generation and distribution even under microscopic observation, including
diaminofluoresceins (DAFC), diaminorhodamines (DAR), diaminoanthraquinone (DAQ),
fluorescent resonance energy transfer (FRET) and the “fluorescent nitric oxide cheletropic
traps” as shown in Table 3 [von Bohlen, et al. 2003, Zhou, et al. 2009].
Inhibition of Nitric Oxide Synthase Gene Expression:
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In following section, we will summarize the progress to date on multimodality imaging of
NO and NOS expression and address future research directions and obstacles for NO/NOS
imaging using biosensors as outlined in Figure 9. For interested readers, basic methods of
NO detection and imaging are given in Appendix in then end of this chapter.





Fluorescence imaging of NO:
2,7-dichlorofluorescin was introduced for NO detection. When it is oxidized by NO, a
fluorescent compound (dichlorofluorescein) is formed. Besides a relatively poor
detection sensitivity (~ 10 μM of NO), 2,7-dichlorofluorescein also has very low
specificity for NO. Later several iron (II) complexes, fluorescently labeled cytochrome c
(a hemoprotein that binds NO selectively), cobalt complexes and fluorescent NO
cheletropic traps (FNOCTs) were introduced. The iron(II) complex of quinoline pendant
cyclam has fluorescence emission at 460 nm, which can be quenched effectively by NO
from NO releasing agents. 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) labeled with
acridine and Fe(II)-(N-(dithiocarboxy) sarcosine)2 complex (Fe(DTCS)2) (Fig.5), was
developed. The addition of an NO-releasing reagent causes a decrease in the
fluorescence signal due to the irreversible binding of NO to the Fe(II).
DAFCs are the most successful and extensively investigated agents for NO imaging. In
vivo imaging of NO in whole living vertebrate with the DAF compounds was reported,
where diaminofluorophore 4-amino-5-methylamino-2′-7′-difluorofluorescein diacetate
(DAF-FM-DA) was used to detect NO production sites in the zebrafish Danio rerio. The
specificity of the fluorescent signal was confirmed by a decrease in animals exposed to a
NO scavenger or a NOS inhibitor, as well as an increase in the presence of a NO donor.
Local changes in NO production in response to stressful conditions could also be
imaged by this agent, suggesting that DAF-FM-DA can be used to monitor changes in
NO production which may have future applications in drug screening and molecular
pharmacology. Best example of NO imaging in vivo with the DAF compound in
zebrafish is quite transparent. The major limitation of the DAF-2 compound is that it is
not specific to NO because it can also react with dehydroascorbic acid (DHA) and
ascorbic acid (AA) to generate new compounds that have fluorescence emission profiles
similar to that of DAF-2T.
Similar to DAF and DAR, rhodamine fluorophore was developed and it was termed “the
DAR”. To image NO in living cells, DARs should be membrane permeable. Therefore, an
ethyl group was introduced into the carboxyl group of DAR-1 (4,5-diamino-N,N,N′,N′tetra ethyl rhodamine), anticipating that the ethyl ester would be hydrolyzed by cytosolic
esterases after permeation through the cell membrane [Kojima et al. 2001].
New class of DAQ compounds of DAFC family called diaminocyanines (DACs) were
prepared with a tricarbocyanine as the NIR fluorophore and o-phenylenediamine as the
NO sensitive fluorescence modulator. It was reported that these compounds can react
faster with NO than the DAFs and the reaction efficiency was at least 50 times higher than
that of DAF-2. Although it was demonstrated that NO can be imaged in isolated rat
kidneys, in vivo imaging of NO using these compounds has not been reported.
A new class of difluoroboradiaza-s-indacene-based agents have been widely used in the
detection of NO in cells or tissues [Huang, et al. 2002a]. One of these compounds,
1,3,5,7-tetramethyl-8-(4′-aminophenyl-N-(2′′-amino)-phenzyl)-difluoroboradiaza-sindacene (TMAPABODIPY), was shown to have high photostability, high sensitivity
(detection limit is 5 nMof NO), and no pH dependency over a wide pH range (see
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Enzyme Inhibition and Bioapplications
Figure 9). Subsequently, a similar compound, 1,3,5,7-tetramethyl-2,6-dicarbethoxy-8(3′,4′diaminophenyldifluoroboradiaza-s-indacene (TMDCDABODIPY), was applied for
real-time imaging of NO in cells [Huang, et al. 2002b].
Emission (color)
Excitation
Reacts with ROS
Tested in
neurotoxic tissues
Neurotoxic
MGD
DAF
DAR
DAQ
~520 nm (green)
~460 nm
Generates
No
~515 nm(green)
~495 nm
No
Yes
>580 nm(red)
~550 nm
-No
460 nm(blue)
~520 nm
No
Yes
--
No
--
No
Table 3. Properties of fluorescent NO-indicators


In copper-based fluorescent compounds copper (II) is coordinated with certain
fluorophores, the base-catalyzed reaction with NO leads to the reduction of the metal
center and the intramolecular nitrosylation of a secondary amine. Based on this
mechanism, a series of fluophore coordinated copper (II) complexes were designed for
NO imaging [Lim, 2007]. Reduction of the Cu (II) core by NO to Cu(I) with nitrosation of
the fluorescent ligand is accompanied by bright fluorescence emission. Any
nitrogen/oxygen reactive species or ascorbic acid do not cause any significant
fluorescence alteration in Cu(II)-based agents. Cu(II) complexes are quite successful in NO
imaging in cell culture but they have not been tested in vivo likely due to the suboptimal
emission wavelength, the potential toxicity, and the poor in vivo stability of the Cu(II)
complex. Recently, several benzimidazole derivatives have been tested in vivo for NO
imaging [Quyang, et al. 2008]. The fluorescent 5′-chloro-2-(2′-hydroxyphenyl)-1Hnaphtho[2,3-d]imidazol and 4-methoxy-2-(1H-naphtho [2,3-d]imidazol-2-yl)phenol can
coordinate with Cu(II) to form a nonfluorescent coordination compound, which can
detect NO production in inflammation with a turn-on fluorescence. NO production was
imaged successfully in both activated murine macrophages and an acute severe hepatic
injury (ASHI) mouse model. Although these agents represent the first successful attempt
of in vivo fluorescence imaging of NO, the actual imaging was performed ex vivo on
frozen tissue slices due to the suboptimal fluorescence emission wavelength as shown in
Figure 10. Much further optimization of these Cu(II)-based agents will be needed in the
future, in particular fluorescence emission in the NIR range and reduction of the toxicity.
Imaging NO/NOS with FRET: FRET agents react with NO covalently a irreversible
sensors for NO. Further, some of the organic dyes can accumulate in subcellular
membranes and emit fluorescence signals in an NO-independent manner which can
cause significant background signal and interfere with the detection of NO. To
overcome these limitations, a NO-selective sensor with the heme domain of soluble
guanylate cyclase (sGC) was developed [Barker, et al. 1999]. The heme domain of the
sGC was labeled with a fluorescent dye and domain changes in the fluorescence
intensity of the dye based on the sGC heme domain's characteristic binding of NO. It
was found that these sensors have fast, linear, and reversible responses to NO
unaffected by other radical species. However, the detection limit is only about 10 μM of
NO which makes less useful.
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Fig. 9. Imaging NO with diamino-aromatic fluorescent compounds. (A) The general reaction
that leads to NO detection by fluorescence with this class of agents.
(B) Fluorescence image of live zebrafish larvae loaded with DAF-FM-DA. Blue arrow, heart;
orange arrow, the cleithrum; purple arrow, the notochord; yellow arrow, the caudal fin.
(C) Distribution of NO in B16F10 tumors grown in the cranial window in mouse. Left:
microangiography using tetramethylrhodamine-dextran; middle: representative
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Enzyme Inhibition and Bioapplications
microfluorography captured 1 h after the loading of DAF-2 in tumors; right: The animals
were treated with NOS inhibitor. Color bar shows calibration of the fluorescence intensity
with known concentrations of DAF- 2T. (D) Left: fluorescent image of endothelial cells with
DAR-4M AM at 10 min after the stimulation. Right: an antagonist of the stimuli strongly
abolished the fluorescence intensity. (E) Confocal laser fluorescence microscopy image of
cells loaded with DAQ and stimulated to produce NO. Left: bright-field; middle: 488 nm
excitation; right: 543 nm excitation. (F) Bright-field and fluorescence images of activated
PC12 cells loaded with TMDCDABODIPY in the presence of arginine. Adapted from
[Huang, et al. 2007a, 2007b].
Fig. 10. Copper-based fluorescence imaging of NO. (A) CuFL detection of NO produced by
iNOS in macrophage cells after different time of incubation. Top: fluorescence images;
Bottom, bright-field images. (B) Fluorescence images of excised liver slices harvested from
live animals after intravenous injection MNIP-Cu. Left: normal mouse liver; center: ASHI
liver; right: Kupffer cell-depleted ASHI liver. Adapted from reference [Quyang, et al. 2008].
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Subsequently, a genetically encoded fluorescent indicator for NO was developed based on
FRET, which can reversibly detects NO with high sensitivity (detection limit of 0.1 nM) and
visualizes the nanomolar dynamics of NO in single living cells on NO induction [JaresErijman, et al. 2003]. However, this approach requires genetic encoding and it generates the
biologically active molecule cyclic guanosine monophosphate (cGMP), which can induce other
cellular responses. The cGMP molecules can bind to both the NO-associated and the No free
probe, resulting in an increase in fluorescence. Therefore, the signal generated is a reflection of
the intracellular concentration of cGMP rather than that of NO per se. Recently, the use of
confocal based spectral imaging with NO-sensitive FRET reporters enabled NO imaging in the
vasculature of intact, isolated perfused mouse lung[St Croix, et al. 2008]. Using calibration
spectra, it is possible to unambiguously separate the cross-talk between overlapping donor
and acceptor emissions. Overall, FRET is a very powerful technique in cell-based experiments
but with only very limited potential applications in animal studies as shown in Figure 11.
3.4 NO bioimaging by bioluminescence and chemiluminescence imaging
The major application of chemiluminescence in NO imaging/ detection lies in gas phase or
liquid phase measurements[Taha, et al. 2003]. The major chemiluminescence activators for NO
imaging include ozone, luminol, and lucigenin. Gas phase chemiluminescence detection of NO
is generally based on the reaction of NO with ozone to produce excited-state nitrogen dioxide.
When it returns to the normal state, light is emitted at about 600 nm wavelength. Such gas
phase detection has been widely used for measuring NO in exhaled breath to detect
inflammatory diseases of the lung. Gas phase chemiluminescence detection has very high
specificity, as well as extreme sensitivity, which can reach parts per billion (ppb). However,
drawbacks include inconvenient instrumentation, high cost, and timeconsuming detection
procedure. To overcome these problems, an optimized chemiluminescence detection system
for NO was developed. For liquid phase chemiluminescence imaging of NO, currently the
luminol/H2O2 detection system is the most useful [Robinson, et al. 1999]. Alkaline luminol can
react with NO to generate luminescence. The presence of H2O2 as an activator can enhance the
sensitivity by about 20-fold. Two approaches have been used for luminol/H2O2-based imaging
of NO. In the first approach, the sample is directly added to the luminol/H2O2 mixture. This
strategy can detect very low levels of NO, which is quite useful for visualizing the production
of NO from cells or organs, but it lacks NO specificity since luminol can react with many free
radical species. In the second strategy, a selective dialysis membrane is placed between the
sample and the luminol/H2O2 mixture. This procedure can offer better specificity yet it suffers
from poor sensitivity, slow response time, and undesirable NO detection limit (μM level).
Besides ozone and luminol, lucigenin is can also be used for NO detection. Lucigenin has been
widely used as a chemiluminescent substrate to monitor vascular superoxide formation. In
particular, lucigenin at a concentration of 5 μM could be used as an indirect probe to estimate
basal vascular NO release. Generally, chemiluminescence is a very sensitive method for NO
imaging/detection. However, the disadvantages such as low specificity and time-consuming
procedure make chemiluminescence not very useful as an independent technique for NO
imaging/detection.
Several other imaging techniques have been investigated for NO detection. As an important
component of molecular imaging, ultrasound is not inherently suitable for direct NO
imaging, although it can be used for evaluating the endothelial function or vasodilation
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Enzyme Inhibition and Bioapplications
caused by NO [Heiss, et al. 2008]. It is worth noting that ultrasound at different frequencies
can stimulate NO production in different contexts such as in endothelial cells, osteoblasts,
and vascular dilation. A few other techniques such as time-resolved photoion and
photoelectron imaging, Hb spectral alteration imaging, laser induced fluorescence detection
(LIF), and calcium amplification imaging have also been studied for NO detection. In
particular, LIF imaging was able to detect the NO release in a single cell, which makes it a
very powerful tool for studying the kinetics of NO release by neuronal cells during
neurotransmission. PET imaging of anesthetized dogs with inhaled radiolabeled 13NO has
been performed to study the in vivo kinetics of NO [McCarthy, et al. 1996]. In future,
molecular imaging may provide more accurate information on NO in intact living systems.
EPR-CT [Yokoyama, et al. 1997], EPR-MRI [Berliner, et al. 2004], and EPRchemiluminescence imaging [Nagano, 2002] have all been attempted and much further
research/optimization will be needed before any potential clinical investigation can be in
place.
3.5 Multimodal bioimaging of NOS gene expression
Initially NOS was considered a sole enzyme to produce NO. Nitric oxide synthase (NOS) is
also under extensive investigation [Crowell et al. 2003]. NO is biosynthesized as three
distinct mammalian NOS isoforms shown in section 1. Neuronal NOS (nNOS or NOS I) and
endothelial NOS (eNOS or NOS III) are constitutively expressed in neuronal and endothelial
cells, respectively and both are referred to as cNOS. Inducible NOS (iNOS or NOS II) is
expressed in cells involved in inflammation, such as macrophages and microglias when
stimulated by cytokines and/or endotoxins. Although cNOS and iNOS do not have a
significant difference in activity (both function through NO), cNOS expression can be
stimulated by Ca2+ whereas iNOS activity is Ca2+-independent. Generally, the NO levels
produced by cNOS in stimulated endothelial and neuronal cells are much lower (in
nanomolar [nM] range) than those generated by iNOS in macrophages (micromolar [μM]
range). Aside from NOS synthesis, recent studies have pointed out that nitrite reduction can
also serve as a possible source of biologically relevant NO [Gladwin, et al. 2005]. Excessive
production of NO is believed to be responsible for various pathophysiologies, while very
low levels of NO are needed to maintain normal physiological conditions. Since the
production of NO is closely associated with the expression of NOS, blocking NOS
expression is a more convenient approach to inhibit the function of NO than intervention
through NO itself.
Molecular imaging refers to the characterization of NO/NOS and measurement of biological
processes at the molecular level [Massoud, et al. 2003]. It takes advantage of traditional
diagnostic imaging techniques and introduces molecular probes to measure the expression
of indicative molecular markers at different stages of diseases. Molecular imaging
modalities include molecular magnetic resonance imaging (MRI), magnetic resonance
spectroscopy (MRS), optical imaging, targeted ultrasound, single photon emission
computed tomography (SPECT), and positron emission tomography (PET). Many hybrid
systems that combine two or more of these modalities are already commercially available
and certain others are under active development. Molecular imaging can give whole body
readout in an intact system at the cost of less workload, inexpansive drug development trial,
and provide more statistically relevant results since longitudinal studies can be performed
Inhibition of Nitric Oxide Synthase Gene Expression:
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145
in the same subjects [Cai, et al. 2008a, 2008b]. In the clinical setting, molecular imaging can
significantly aid in early lesion detection, patient stratification, and treatment monitoring,
which can allow for much earlier diagnosis, earlier treatment, better prognosis, and
individualized patient management. To date, the multimodal imaging modalities for
visualization of NO include optical imaging (fluorescence and chemiluminescence), electron
paramagnetic resonance (EPR) imaging, and MRI. Meanwhile, NOS has been mainly
investigated using optical and PET imaging. In following section, we describe recently
developed multimodal techniques in their infancy.
3.5.1 Optical imaging of NOS expression
Imaging enzyme expression and function is quite challenging. Compared to the vast
number of literature reports on NO imaging, much less effort has been directed toward NOS
imaging. Briefly, the major modalities for NOS imaging include optical imaging, PET, and
some other techniques such as electron microscopy. Historically, the study of NOS
physiology was constrained by the lack of suitable probes to detect NOS in living cells or
animals. In one pioneering study, a fluorescent iNOS inhibitor, pyrimidine imidazole FITC
(PIF), was developed and investigated for microscopy of iNOS in living cells [Panda, et
al.2005]. It is an isoform-selective molecule with high affinity to iNOS and the binding is
essentially irreversible, which allowed for iNOS imaging in many cell types as well as
freshly obtained human lung epithelium. This study indicated that fluorescent probes (e.g.,
PIF) can be valuable for studying iNOS cell biology and understanding the pathophysiology
of diseases that involve dysfunctional iNOS expression. In another report,
immunofluorescence staining was performed to evaluate nNOS expression on left inferior
alveolar nerve (IAN) injury in ferrets [Davies, et al. 2004]. A possible translocation of nNOS
protein from the cell body to the site of nerve injury was proposed. Lastly, some
ruthenium(II) and rhenium(I)-diimine wires were reported to be capable of binding to iNOS
in vitro to generate NIR fluorescence (710 nm emission), which may potentially be useful as
a fluorescent imaging probe for iNOS [Dunn, et al. 2005]. The inflammatory modulating
effects of iNOS on laser therapy were studied using bioluminescence imaging (BLI)
[Moriyama, et al. 2005]. In this study, the efficiency of different wavelengths laser therapy at
different wavelengths in modulating iNOS expression was determined using transgenic
animals with a luciferase gene driven by the iNOS gene. On induction of inflammation
followed by laser treatment, imaging of iNOS expression was performed at various times by
measuring the bioluminescence signal. When compared with the nonirradiated animals, a
significant increase in the bioluminescence signal was observed after laser irradiation which
demonstrated that iNOS expression can be detected by noninvasive BLI. Comparing the two
strategies for NOS imaging with optical techniques, many fluorescently labeled NOS
inhibitors have charges which makes them not cell membrane permeable. NOS-controlled
luciferase expression appears to be a preferable choice, yet much further optimization will
be needed in future studies. Generally speaking, the limited penetration of light in tissue
renders optical imaging only suitable for a limited number of scenarios (e.g., superficial
regions-of-interest and endoscopy-accessible tissues). Another drawback of optical imaging
is that it cannot give accurate quantitative analysis of the imaging result. Combination of
quantum dot optical and radionuclide-based imaging techniques (e.g., PET) may
significantly help in this aspect [Cai, et al.2007a, 2007b].
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Enzyme Inhibition and Bioapplications
3.5.2 PET imaging of NOS expression
Small animal PET scanners have very good spatial resolution (1 mm) and they are also
becoming increasingly widely available. PET does not have a tissue penetration issue, it
is highly quantitative, and it can be used for both diagnosis and treatment monitoring. To
date, PET imaging of NOS expression has been focused on the clarification of nNOS
function in neurological diseases [Pomper, et al. 2000, Zhang, et al. 1997]. PET imaging of
nNOS was initially investigated with a 11C-labeled nNOS inhibitor, S-methyl-Lthiocitrulline (MTICU) (FIG 11, 12) [Zhang, et al. 1997]. Biodistribution of 11C-MTICU
gave a brain-to-blood ratio of 1:2 at 30 min postinjection. Uptake in the cerebellum (high
nNOS expression) was 20% higher than that of the cortex or the brain stem (low nNOS
expression). Blockage using 1 mg/kg of MTICU was able to reduce the uptake in the
cerebellum and the cortex, but not in the brain stem. Study demonstrated the potential of
11CMTICU as a potentially useful tracer in determining nNOS levels in vivo. In another
report, two nNOS-selective PET tracers, AR-R 17443 [N-(4-(2-(phenylmethyl) (methyl)amino)ethyl)phenyl)-2-thiophenecarboximidamide)] and AR-R 18512 [(N(2-methyl1,2,3,4-tetrahydroisoquinoline-7-yl)-2 thiophenecarboximidamide)] were synthesized and
evaluated in rodents and primates [Pomper et al. 2000]. Another strategy has been
employed for iNOS imaging using antisense oligonucleotide with hybridization
properties for iNOS mRNA using RT-PCR. The oligonucleotide was then labeled with 18F
(FIG 12) [de Varies, et al. 2004]. Cellular uptake or efflux showed no selectivity for iNOS
expressing cells. Thus PET imaging of NOS expression has not been very successful which
is likely due to several reasons. First, the expression of NOS is in the cytoplasm. Although
the radiolabeled NOS inhibitors usually have high affinity for NOS, they may not
permeate through cell membrane efficiently which dramatically affects the tracer uptake.
Second, the specificity of these NOS inhibitors may not be high enough for imaging
applications; many of them can undergo nonspecific adsorption to other proteins such as
albumin. Third, the stability of those tracers in vivo is a major concern. Uptake in the
tumor or other targeted organs may not truly reflect the distribution of the intact tracer.
Lastly, these NOS inhibitors are small molecules. Although replacing the 12CH3 with
11CH3 will not change the chemical structure, in many cases this cannot be achieved.
Recently, a novel NOS inhibitor design strategy, termed the “anchored plasticity
approach,” may give rise to novel NOS inhibitors for PET imaging applications in the
future [Garcin et al. 2008].
3.5.3 Imaging NOS expression with other techniques
Besides optical and PET imaging, other techniques have been explored for NOS-related
research, mainly electron microscopy and immunohistochemistry [Heinrich, et al. 2005,
Lajoix, et al. 2006]. Electron microscopy was applied to visualize the subcellular localization
of NOS in different cell organelles using gold particles conjugated with anti-NOS antibodies,
which gave exquisite distribution information of NOS in cells. Immunohistochemistry also
uses anti-NOS antibodies, easier to perform than electron microscopy yet the resolution is
not as high. Both approaches are invasive and not suitable for in vivo imaging; however, the
experimental findings are typically quite reliable and thus they can serve as ex vivo
validation for in vivo imaging studies.
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Fig. 11. (on left) Imaging NO with FRET. (A) Basic principles of FRET between CFP and
YFP. CFP: cyan fluorescent protein; YFP: yellow fluorescent protein. (B) Pseudocolor images
of the CFP/YFP emission ratio of the cell before (1570 s) and after addition of NOC-7, an
NO-donating reagent that releases 2 mol of NO per mole. Adapted from reference [Hong et
al. 2009]
Fig. 12. (on right) Several NOS inhibiters and an antisense oligonucleotide for iNOS mRNA
have been radiolabeled for PET imaging of NOS expression.Adapted from reference [Hong,
et al. 2009]
4. Biological applications of NO imaging
Still development and applications of fluorescent agents in cell imaging and NO sensitive
MRI contrast agents for in vivo physiological fMRI are in infancy. MRI imaging of NO based
on nitrite concentration can detect physiological changes such as cerebral blood volume
change in nonischemic spontaneously hypertensive rats [Caramia et al. 1998], increase in
tumor blood flow and partial oxygen pressure after NO donor administration [Jordan et al.
2000], NO production alteration in humans after a 30-min exposure to a 1.5 T magnetic field
[Sirmatel, et al. 2007]. Interestingly, high magnetic strength can cause NO concentration
fluctuations during a MRI scanning of NO-related diseases.
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The major advantages of MRI include high resolution, good soft tissue contrast, wide
availability, sufficiently large field-of-view, and the ability to provide both functional and
anatomical information. Nitric oxide bioimaging has emerged as technique to visualize the
intracellular metabolism based on redox reactions, mitochondrial oxidation and nitric oxide
synthase enzyme NOSi [Anbar, 2000, Zhao, et al. 2009]. Recently, nitric oxide was invented
as active mechanism to play role in vascular physiology in lungs, kidney, heart, brain, sex
organs. Endothelial cell, smooth muscle cells, oxygenation and perfusion, hypoxia, ischemia
are the possible targets of nitric oxide biotransformation. These physiological changes are
likely to be visualized by nitric oxide bioimaging in real time manner and reviewed in the
following sections.
4.1 Cardiovascular and endothelial cell injury
DAF-FM is useful tool to visualize the temporal and spatial distribution of intracellular NO.
Endogenous ATP plays a central role in HTS-induced NOS in endothelial cell injury.
Endothelial cNOS, a calmodulin-dependent enzyme is critical for vascular homeostasis and
makes detectable basal level of NO production at low extracellular Ca2+. cNOS expression
can be stimulated by Ca2+ while iNOS activity is Ca2+-independent. Actin microfilaments in
primary artery endothelial cells (PAEC) regulates L-arginine transport and this regulation
can affect NO production by PAEC, DAR-4M may be useful for bioimaging of samples that
have strong autofluorescence [Lai, et al. 1994, Kojima, et al. 2001]
4.2 Apoptosis in cells
Apoptosis plays a key role in many pathological circumstances, such as neurodegenerative
diseases. In these processes, the involvement of nitric oxide (NO) has been well established,
and the ability of NO to exert cellular damage due to its reactive oxidative properties is
perhaps the primary neurotoxic mechanism. Emerging evidences indicate that apoptosis
contributes to neuronal death to explain neurodegeneration [Fujii et al. 1997]. The major
biochemical change in activation of the cystein protease (caspase-3) was reported to execute
apoptosis in the central nervous system disorders such as stroke, spinal cord trauma, head
injury and Alzheimer’s disease [Green,1998, Nagata,1997 ]. Although numerous techniques
have been described to evaluate apoptosis, these approaches involve invasive techniques
and cannot provide detailed information about apoptosis in vivo [Anbar, 2000].
4.3 Renal and cardiovascular imaging
A new approach of irreversible responsive PARAmagnetic Chemical Exchange Saturation
Transfer (PARACEST) for MRI contrast agents constituted a new type of agent for molecular
imaging. A novel PARACEST MRI contrast agent, Yb(III)-(1,4,7,10-tetraazacyclododecane1,4,7-triacetic acid)-orthoaminoanilide (Yb-DO3A-oAA), was developed to image nitric
oxide (NO). The agent exhibited two CEST effects at -11 ppm and +8 ppm chemical shifts,
which were assigned to chemical exchange from amide and amine functional groups,
respectively. This responsive PARACEST MRI contrast agent indicated the change in the
presence of NO and O(2), which caused an irreversible disappearance of both PARACEST
effects from MR images. This report highlighted the advantages of irreversible MRI contrast
agents and demonstrated that large changes in PARACEST can be used to in vivo
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biomedical applications in molecular imaging [Liu, et al. 2007]. In other study, the clinical
detection of evolving acute tubular necrosis (ATN) and differentiating it from other causes
of renal failure was performed. Herein, sodium magnetic resonance imaging (23Na MRI) was
applied to study the early alteration in renal sodium distribution in rat kidneys 6 h after the
induction of ATN combined with inhibition of nitric oxide and prostaglandin synthesis.
Hence, 23Na MRI non-invasively quantified changes in the corticomedullary sodium
gradient in the ATN kidney during morphologic tubular injury [Maril, et al. 2006]. Other
newer application of 19F NMR spin-trapping technique for in vivo *NO detection was
employed to elucidate the significance of *NO availability in animal models of hypertension.
In vivo *NO-induced conversion of the hydroxylamine from fluorinated nitrosyl nitroxide
(HNN) to the hydroxylamine of the iminonitroxide (HIN) was evidenced in hypertensive
ISIAH and OXYS rat strains [Bobko, et al. 2005]. The NMR detected positive levels of
nitrite/nitrate for in vivo evaluation of *NO production and provided the basis for in vivo
*NO imaging [Kojima, et al. 2001, Kojima, et al. 2001].
The cardiovascular ischemia in tissues generates the nitric oxide (NO). It is an enzymeindependent mechanism of NO generation and more pathogenesis. So, the NO formation in
mice after cardiopulmonary arrest could be imaged. Real-time measurement of NO
generation was performed by detection of naturally generated NO-heme complexes in
tissues using L-band electron paramagnetic resonance (EPR) spectroscopy. Measurements of
NO generation were imaged on the intact animal at the levels of the head, thorax, and
abdomen within 3 hours [Ueno, et al. 2002]. Very recently, nitric oxide was recognized as
biomarker of vascular dysfunction in renal, clitoris and penile sex organs to detect the
physiology [Ockalli, et al. 1999, Kakiailatu, 2000, Burnett, 2002, Barouch, et al. 2002,
Hillebrandt, et al. 2009, Ciplak, et al. 2009, Gragasin, et al. 2004, Richards, et al. 2003]. The
NO complexes were found to have maximum levels in lung, heart, and liver by threedimensional spatial mapping of the NO complex in the intact animal subjected to
cardiopulmonary arrest. The images also confirmed the maximum NO formation in the
lungs, heart, and liver [Kuppusamy, et al. 2001].
4.4 Calcium-imaging and NO-imaging
The combined real-time bioimaging of calcium and nitric oxide with continuous
electrophysiological intracellular recording has gained enormous interest. For intracellular
monitoring by calcium imaging in cells, Fura-2 is a calcium-sensitive fluorescent probe.
Using Fura-2, intracellular free calcium ion concentration ([Ca2+]i) of single neurons was
measured [Richards, et al. 2003]. Calcium-imaging with Fura-2 was used to monitor and
modulate the intracellular Ca2+ evoked by NMDA and AMPA in cultured hippocampal
pyramidal cells [Keelan, et al. 1999] Calcium and NO play role in excitotoxic mitochondrial
depolarization in the hippocampus [Berkels, et al. 2000]. The combination of NO bioimaging
with calcium imaging may distinguish the temporal rise in NO and calcium to suggest that
NO is being produced as a consequence of Ca2+ enhancement and Ca2+ induced NO
Synthase activity). Recently, imaging intracellular calcium imaging was achieved together
with NO imaging by using two monochromators DAF-2 and Fura-2 AM as excitation light
source. The DAF-2 excited at 485 nm and Fura-2 AM excited at 340 nm [Berkels, et al. 2000].
The emission maximum of fura-2 is about 510 nm different while the emission maximum of
DAF-2 is about 515 nm. By using dichroic or multichroic mirrors calcium and NO levels can
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be measured simultaneously [Pittner, et al. 2003]. Other example of monochromator
complex is DAQ and DAR-4M AM. These are calcium sensitive non-neurotoxic dyes with
unique fluorescent patterns of DAQ (excitation 520 nm; emission 580 nm) and DAR-4M
AM (excitation  550 nm; emission  580 nm). These can also combine with the calciumsensitive dye Fura-2 as potential tools in fluorescence microscopy.
4.5 Kupffer cell NO bioimaging
A new mechanism was reported in nonparenchymal cells based on Bcl-2/adenovirus EIB
19-kDa interacting protein 3 (BNIP3), a cell death-related member of the Bcl-2 family, was
upregulated in vitro and in vivo by nitric oxide (NO) downregulation of BNIP3 gene
expression as one mechanism of hepatocyte cell death and liver damage. Authors proposed
that inflammatory stresses can lead to the modulation of BNIP3 [Metukuri, et al. 2009].
4.6 Combining electrophysiology and NO-imaging
The calcium-imaging with electrophysiological recording was excitement in the
hippocampus imaging art because of calcium accumulation in hippocampus pyramidal cells
during synaptic activation [Regehr, et al. 1989]. Now a day, calcium-imaging combined with
electrophysiological stimulation and recording has emerged as present state of art of
calcium-signaling and intracellular free calcium ion concentration [Ca2+]i of single neurone.
Moreover, little is understood how neuronal signals stimulate NO production. Potential
applications of fluorescent NO indicators in bioimaging are to monitor NO in response to
pharmacological manipulation using NO-donors, NOS substrate, and NOS or NOS isoform
inhibitors as enhancers of NO production in vivo and in vitro [Moore, et al. 1997, Southan,
et al. 1996]. Other emerging techniques of bioimaging by NO inhibitor are targeting the
differential cofactor requirements of the various isoforms; pharmacological agents targeting
the differential substrate requirements of cell expression of various isoforms of the NOS
[Salerno, et al. 2002]. However, this approach suffers due to lack a marked isoform
selectivity or direct affecting the arginine transport into cells, like NG-methyl-L-arginine (LNMA). Other example of NOS inhibitors such as 7-nitroindazole (7-NI), might have
unexpected side-effects in neuronal tissues, since 7-NI interferes with monoamine oxidase
[Castagnoli, et al. 1999, Desvignes, et al. 1999]. However, selective NOS-inhibitors together
with fluorescent NO-imaging appear as useful tools for the investigation of the biological
functions of the different NOS-isoforms in neuronal tissues [Chen, et al. 2001, Brown, et al.
1999]. The combined NO-imaging with in vitro electrophysiology methods visualize the NO
generation and NO distribution with localization of NO induced by neurotransmitter
stimulations, e.g., NO formation after neuronal stimulation with NMDA, in normal synaptic
transmission, in paired-pulse facilitation, in long-term depression or LTP. Thus, in a single
preparation, the spatial distribution of NO after induction of hippocampal LTP has been
evaluated [Schuppe, et al. 2002]. Main unsolved issues of precise time-point of NO release
and selective NOS isoform-inhibitors in neuronal activity still remain to investigate before
the combined fluorescent NO-imaging and electrophysiology method becomes an
acceptable bioimaging tool. In near future, the researchers will thrive to combine
electrophysiology with simultaneous calcium-imaging and NO-imaging to get better
answers of the role and interaction of calcium and NO in physiological and pathological
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neuronal processes with continuous monitoring the calcium and NO by dichroic or
multichroic mirrors in synaptic plasticity.
4.7 Alveolar cells
Human alveolar macrophages and alveolar epithelium is the site of nitric oxide synthase-2
(NOS-2) messenger ribonucleic acid (mRNA) and protein expression in alveolar wall in lung
of patients with pulmonary malignancies. In the process of NO production, proinflammatory cytokines interleukin (IL)-1β, tumour necrosis factor (TNF)-, interferon
(IFN)- or lipopolysaccharide (LPS) play significant role to induce the effect of human
surfactant protein-A (SP-A) on IFN--mediated NOS-2 mRNA expression [Pechkovsky, et al.
2002].
4.8 Endothelial cells
Kojima et al. 2001, reported NO bioimaging in cultured bovine aortic endothelial cells. The
cultured bovine aortic endothelial cells were incubated with DAF-FM DA for 1 h for dye
loading. After stimulation with bradykinin, the fluorescence intensity in the cells increased
and that in the cytosol increased more than that in the nucleus. NO is produced in the
cytosol, where NOS exists [von Bohlan, et al. 2002]. This observation implies that little of the
produced NO diffuses into the nucleus or if it does diffuse into the nucleus little of it is
oxidized there. The augmentation of the fluorescence intensity was suppressed by an NOS
inhibitor. In conclusion, DAF-FM is a useful tool for visualizing the temporal and spatial
distribution of intracellular NO. Kimura et al.2001, examined the effects of acute glucose
overload on the cellular productivity of NO in bovine aortic endothelial cells (BAEC) using
DAF-2. ATP induced an increase in NO production, assessed by DAF-2, mainly due to Ca2+
entry as shown in Figure 13. In contrast, the ATP-induced increase in DAF-2 fluorescence
was impaired by glucose overload. The results indicate that glucose overload impairs NO
production via the O2--mediated attenuation of Ca2+ entry. In addition, Kimura et al. 2000,
examined in detail the mechanism by which mechanical stress induces NOS in endothelium.
Hypotonic stress (HTS) induced ATP release, which evoked Ca2+ transients in BAEC. HTS
also induced NOS, assessed by DAF-2 fluorescence. The results obtained indicate that
endogenous ATP plays a central role in HTS-induced NOS in BAEC. Broillet et al. 2001,
reported that Ca2+, Mg2+, or incident light promoted the reaction of DAF-2 and NO.
However, Nagano et al. 2002, probed that the enhancement of fluorescence intensity was
caused not by increasing the reaction rate between DAF-2 and NO but by promotion of NOreleasing rate of NO donors. Endothelial NOS, a Ca2+/calmodulin-dependent enzyme, is
critical for vascular homeostasis [Suzuki, et al. 2002]. To determine the signaling pathway
for endogenous eNOS, Lin et al. 2000, directly measured NO production in bovine
pulmonary artery endothelial cells (PAEC). Endothelial cells grown in a monolayer produce
very low levels of NO which are below the detection range of traditional assays. Therefore,
they employed DAF-2 DA to measure NO production in real-time. PAEC had detectable
basal NO production which increased slightly after thapsigargin treatment in the presence
of low extracellular Ca2+. In contrast, a large increase in NO production was detected in the
presence of 4 mM extracellular Ca2+, suggesting that capacitative Ca2+ entry regulates wildtype eNOS activity. Zharikov et al. 2001, investigated possible involvement of the actin
cytoskeleton in the regulation of the L-arginine/NO pathway in pulmonary artery
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endothelial cells (PAEC). DAF-2 DA was used for detection of NO in PAEC. They conclude
that the state of actin microfilaments in PAEC regulates L-arginine transport and that this
regulation can affect NO production by PAEC. Berkels et al. 2000, presented a new method
to measure intracellular Ca2+ and NO simultaneously in endothelial cells. The method
makes it possible to follow intracellular Ca2+ and NO distributions online and is sensitive
enough to monitor changes of NO formed by the constitutive endothelial NOS. Franz et al.
2000, reported DAR-4M AM applied to imaging of NO in bovine aortic endothelial cells
using aminotroponiminates[Franz et al. 2000]. DARs are more photostable than DAFs. The
fluorescence in cells was observed after stimulation with bradykinin (see Figure 14), which
raises the intracellular Ca2+ level and thereby activates NOS, and this increase was
suppressed by addition of an NOS inhibitor. DAR-4M should be useful for bioimaging of
samples that have strong autofluorescence in the case of 490 nm excitation in which the
intracellular pH may fall below 6. Hiratsuka et al. 2009, reported in vivo visualization of
nitric oxide and intracellular inflammatory interactions among platelets, leukocytes, and
endothelium following hemorrhagic shock and reperfusion.
Fig. 13. Bright-field and fluorescence images of cultured bovine aortic endothelial cells
loaded with DAR-4M AM. The upper images are the bright-field, the fluorescence, and the
fluorescence ratio images at the start of measurement, respectively. The middle and the
lower images are fluorescence ratio images at the indicated times after the start of
measurement. The fluorescence ratio images indicate the ratio of the intensity to the initial
intensity at the start. Adopted from reference Kimura, et al. 2001
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Fig. 14. Fluorescence images of a rat brain slice loaded with DAF-2 DA. The fluorescence
intensity is shown in pseudocolor. (Reproduced from reference Nagano, et al. 2002)
4.9 Smooth muscle cells
DAF-2 DA was applied to the imaging of NO in cultured rat aortic smooth muscle cells by
using a fluorescence microscope equipped with fluorescence filters for fluorescein
chromophores. Confocal laser scanning microscopy indicated that fluorescence was emitted
from the whole cell body, including the nucleus. This implies that DAF-2 regenerated
intracellularly by esterase is distributed throughout the whole cell. There was no indication
of any cell damage caused by loading the dye. The results show that the fluorescence
intensity in endotoxin- and cytokine-activated cells increased owing to DAF-2 T production
from DAF-2 by reaction with NO. The fluorescence did not increase in inactivated cells,
which did not produce NO. The probe is very useful for bioimaging of NO in cultured rat
aortic smooth muscle cells. Itoh et al. detected DAF-FM T using reversed-phase highperformance liquid chromatography with a fluorescence detector [Itoh et al. 2000]. This
sensitive method enabled them to detect the spontaneous and substance P-induced NO
release from isolated porcine coronary arteries, both of which were dependent entirely on
the NOS activity in vascular endothelial cells. Furthermore, they obtained fluorescence
images of cultured smooth muscle cells of the rat urinary bladder after loading with DAFFM DA. In the cells pretreated with cytokines, the fluorescence intensity increased with time
after DAF-FM loading. In recent studies on imaging of nitric oxide in nitrergic
neuromuscular neurotransmission suggested the new insight of molecular mechanisms in
animal organs [Thatte, et al. 2009, Perrier, et al. 2009, Di Cesare Mannelli, et al. 2009].
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4.10 Brain and neuronal behavior
Reif et al. 2009, reported the influence of functional variant of neuronal nitric oxide synthase on
impulsive behaviors in humans. The biological functions of NO in the neuronal system remain
controversial. Using DAF-2 DA for direct detection of NO, Nagano et al. 2002, examined both
acute rat brain slices and organotypic culture of brain slices to ascertain NO production sites.
The fluorescence intensity sensitive to Ca++ in the CA1 region of the hippocampus was
augmented, especially after stimulation with NMDA, in acute brain slices. This NO production
in the CA1 region was also confirmed in cultured hippocampus. Glutamate and NO were
important mediators, since antagonists of both N-methyl-D-aspartate (NMDA) and NOS
inhibitors were effective in attenuating neurotoxicity. This is the first direct evidence of NO
production in the CA1 region as shown in Figure 4. There were also fluorescence cells in the
cerebral cortex after stimulation with NMDA. Imaging techniques using DAF-2 DA should be
very useful for the clarification of neuronal NO functions. Calcium sensitive fluorescent probe
can monitor intracellular calcium by FURA-2. Combined calcium and NO together can be
imaged by excitation light source (DAF-2) excited at 485 nm, and Fura-2 AM excited at 340 nm
using dichroic or multichroic mirrors.DAQ, DAR-4M AM are other choices. NOS-II may be
involved in Alzheimer’s disease as b-amyloid induces NOS-II expression in astrocytes
Astrocytes which which can be potentiated by cytokines [Vodovotz et al. 1996].
DAF-FM DA was also applied to imaging of NO generated in rat hippocampus slices by
exposure to an aglycemic medium [Reif et al. 2009]. NO production was observed mainly in
the CA1 area and was dependent on the concentration of O2. During exposure to an anoxicaglycemic medium, NO was hardly produced while marked elevation of intracellular Ca2+
was observed. Production of NO increased sharply as soon as the perfusate was changed to
the normal medium. These results suggest that NOS is activated after reperfusion rather
than during ischemia. ROS and NO are important participants in signal transduction that
could provide the cellular basis for activity-dependent regulation of neuronal excitability
[Yermolaieva, et al. 2000]. In young rat cortical brain slices and undifferentiated PC 12 cells,
paired application of depolarization/agonist stimulation and oxidation induces long-lasting
potency of subsequent Ca2+ signaling that is reversed by hypoxia. This potency critically
depends on NO production and involves cellular ROS utilization. Using the fluorescent dye
DAF-2, NO production was measured in PC 12 cells during depolarization or histamine
application [Yermolaieva, et al. 2000]. Application of 100 µM histamine induced a significant
increase in NO production. The crucial role for NO in oxidative potentiation of Ca2+
signaling triggered by pairing of electrical/agonist stimulation and oxidation demonstrated
here provides an insight into the cellular mechanisms involved in neuronal developmental
plasticity. NO could influence its effectors by stimulating the cGMP-dependent signaling
pathway and/or by acting as a weak radical. Perrier et al. 2009, reported the effect of
uncoupling endothelial nitric oxide synthase on calcium homeostasis in aged porcine
endothelial cells in heart. In other report, Sergeant et al. 2009, reported the spontaneous Ca2+
waves in rabbit corpus cavernosum were modulated by nitric oxide. Role of cGMP was
crucial in regulation of calcium dependent neuroactive energy changes.
4.11 Ion channels
Most voltage-gated Na+ channels are almost completely inactivated at depolarized
membrane potentials, but in some cells a residual Na+ current is seen that is resistant to
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inactivation. The biological signaling mechanisms that regulate the persistence of Na+
channels are not well understood. Ahern et al. 2000, showed that in nerve terminals and
ventricular myocytes, NO reduces the inactivation of Na+ current. This effect was
independent of cGMP and blocked by N-ethylmaleimide. Thus, ROS act directly on the
channel or a closely associated protein. Application of ionomycin to raise the intracellular
Ca2+ concentration in myocytes activated NOS. The NO produced in response to ionomycin
was detected with DAF-2 DA. They concluded that NO is a potential endogenous regulator
of persistent Na+ current under physiological and pathophysiological conditions. Choi et al.
2000, reported that the NMDA receptor (NMDAR)-associated ion channel is modulated not
only by exogenous NO, but also by endogenous NO. They examined inhibition of NMDAR
responses by endogenous NO to determine the underlying molecular mechanism. For this
purpose, HEK 293 cells transfected with nNOS were transiently transfected with wild-type
NR1/NR2A or mutant NR1/NR2A (C399A) receptors. NMDA responses were monitored
by digital Ca2+ imaging with fura-2. Production of endogenous NO by these HEK-nNOS
cells was measured with DAF-2. It was concluded that endogenous S-nitrosylation may
regulate ion channel activity.
4.12 Fertilization and developmental biology
Kuo et al. 2000, demonstrated that NO is necessary and sufficient for egg activation at
fertilization. The early steps that lead to the rise in Ca2+ and egg activation at fertilization are
unknown but are of great interest, particularly with the advent of in vitro fertilization
techniques for treating male infertility and whole-animal cloning by nuclear transfer. They
showed that active NOS is present at high concentration in sperm after activation by the
acrosome reaction. An increase in nitrosation within eggs is evident seconds after
insemination and precedes the Ca2+ pulse of fertilization. They used DAF-2 DA for detection
of NO. It was concluded that NOS- and NO-related bioactivity satisfy the primary criteria of
an egg activator, i.e. they are present in an appropriate place, active at an appropriate time,
and necessary and sufficient for successful fertilization.
4.13 Inflammation
NO plays important roles in inflammatory processes. Lopez-Figueroa et al. 2000, examined
whether changes in NOS mRNA expression lead to similar temporal and anatomical
changes in NO production in an experimental model of CNS inflammation. iNOS mRNA
expression was analyzed 2, 4, 6, and 24 h after intracerebroventricular (icv) injection of
interleukin-1β or the vehicle. Increased expression of iNOS mRNA was observed
surrounding the microinjection site and meninges. Using DAF-2 DA for the direct detection
of NO production, they observed a significant increase in NO production after 4 and 6 h.
They conclude that increase in iNOS mRNA following icv administration of IL-1β leads to
increases in NO production. They proposed that the DAF-2 DA method can be used as a
potential marker in the diagnosis of CNS inflammation. MRI of NO is very useful because
NO is available at high concentrations in millimolar levels in immune-related diseases (e.g.,
inflammation).Terashima et al. 2010, reported in vivo detection of iNOS expression in
murine carotid lesions by biuoluminescence may provide a valuable approach for
monitoring vascular gene expression and inflammation in small animal models. The phasic
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nature of inflammation is also associated with a decrease in NOS activity. NOS inhibitors
are effective at alleviating pain and have disease modifying effects. Induction of NOS-II is
seen in models of septic shock. NOS-II inhibitors (dexamethasone) prevent hypotension and
decrease in PO2.
Fig. 15. Optical imaging of iNOS expression. (A) Fluorescence imaging of iNOS in living
cells. Left: cells were induced for iNOS expression; Right: cells were not induced for iNOS
expression. (B) Bioluminescence signal from the knee joints (circle) indicates iNOS
expression following inflammation induction. Adapted from reference [Panda, et al. 2000].
4.14 Adrenal zona glomerulosa
Adrenal zona glomerulosa (ZG) cells do not contain NOS. Hanke et al. 2000, conferred
endothelial NOS activity upon adrenal ZG cells through transduction with a recombinant
adenovirus encoding the endothelial NOS gene (AdeNOS) to determine the effect of
endogenous NO on aldosterone synthesis. AdeNOS-transduced cells exhibited DAF-2 DA
fluorescence, which was blocked by pretreatment with an NOS inhibitor. In conclusion,
adenovirus-mediated gene transfer of eNOS in ZG cells results in the expression of active
endothelial NOS enzyme, and this endogenous NO production by ZG cells decreases
aldosterone synthesis.
4.15 Bone marrow stromal cells
Gorbunov et al. 2000, reported that using the reverse transcription-polymerase chain
reaction and immunofluorescence analysis of murine bone marrow stromal cells after -
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irradiation doses of 2-50 Gy stimulated the expression of iNOS. The activation of iNOS was
accompanied by an increase in the fluorescence of DAF-2 and accumulation of 3nitrotyrosine within cellular proteins in a dose-dependent manner.
4.16 Mitochondria
NO has been implicated in the modulation of mitochondrial respiration, membrane
potential, and subsequently apoptosis. Although the presence of mitochondrial NOS was
reported while there is no direct evidence in vivo of the presence of NO within
mitochondria. Using DAF-2 DA, Lopez-Figueroa et al. observed NO production in PC 12
and COS-1 cells by conventional and confocal fluorescence microscopy [Lopez-Figueroa, et
al. 2000]. The subcellular distribution of NO production is consistent with the presence of a
mitochondrial NOS.
4.17 Retina
In the retina, NO functions in network coupling, light adaptation, neurotransmitter receptor
function, and synaptic release. Neuronal NOS is present in the retina of every vertebrate
species so far investigated. However, although nNOS can be found in every retinal cell type,
little is known about the production of NO in specific cells or about the diffusion of NO
within the retina. Blute et al. 2000, used DAF-2 to image real-time NO production in turtle
retina in response to stimulation with NMDA. In response to NMDA, NO was produced in
somata in the ganglion cell and inner nuclear layers, in synaptic boutons and processes in
the inner plexiform layer, in processes in the outer plexiform layer, and in photoreceptor
inner segments. They concluded that NO may function at specific synapses, modulate gene
expression, or coordinate events throughout the cell. Matsuo et al. 2000, reported that basal
NO production is enhanced by hydraulic pressure in cultured human trabecular cell. He
measured the intracellular NO level in real-time using DAF-2.
4.18 Drosophila
Wingrove and O'Farrell 1999, reported that Drosophila utilizes components of the NO/cGMP
signaling pathway to respond to hypoxia. Hypoxic exposure rapidly induced exploratory
behavior in larvae and arrested the cell cycle. These behavioral and cellular responses were
diminished by an inhibitor of NOS and by a polymorphism of cGMP-dependent protein
kinase. Regions of the larvae that specialize in responding to hypoxia should express
significant levels of the activities involved. They used DAF-2 DA to define possible foci of
function. DAF-2 DA stained the pouch of tissue surrounding the anterior spiracles as well as
neuronal-like processes within the pouch. This staining near the openings of the tracheal
system is interesting because the location is consistent with a possible role in governing the
opening of the spiracles to increase access to oxygen. DAF-2 DA is useful for clarification of
response mechanisms to hypoxia.
4.19 Plants
Leaves and callus of Kalanchoe daigremontiana and Taxus brevifolia were used to investigate
NO-induced apoptosis in plant cells [Huang, et al. 2000]. NO production was visualized in
cells and tissues with DAF-2 DA. The NO burst preceded a significant increase in nuclear
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DNA fragmentation and cell death. L-NMMA significantly decreased NO production and
apoptosis in both species. Pedroso et al. 2000a, 2000b concluded that NO is involved in DNA
damage leading to cell death and proposed a potential role of NO as a signal molecule in
these plants. Foissner et al.2000, used DAF-2 DA, in conjunction with confocal laser
scanning microscopy, for in vivo real-time imaging of an elicitor-induced NO burst in
tobacco. A growing body of evidence suggests that NO, an important signaling and defense
molecule in mammals, plays a key role in activating disease resistance in plants, acting as a
signaling molecule and possibly also as a direct antimicrobial agent. The results revealed
additional similarities between plant and animal host responses to infection.
5. NO/NOS bioimaging: Future prospects
The existing techniques based on quenching spin trap or paramagnetic heavy metals pose a
concern of contrast agent toxicity with limitations of event capturing to generate NO image
of isolated cells by EPR or in vivo body image by MGD enhanced MRI. However, unique
role of short lived NO molecule in the cell is peculiar to give insight of rapid ionic
regulatory mechanisms such as calcium, sodium dependent signaling events including
dietary effects, antimicrobial therapy on inflammation, angiogenesis and hypoxia reported
in last three years [Manuel, et al. 2002, Palombo, et al. 2009]. To date, the main imaging
modalities for visualization of NO include fluorescence and bio/chemiluminescence optical
imaging, electron paramagnetic resonance (EPR) imaging, and MRI. cNOS has been mainly
investigated using optical and PET imaging. Paper has presented a progress to date on
multimodality imaging of NO and addressed the future research directions and obstacles of
NO imaging. Other major developments are expected in design of robust imaging hardware
with improved image generating capability and highly sensitive to the rapid changes of
NO/NOS molecular EPR signal and MRI frequency with time. In this direction, active
research efforts suggest the development and availability of less toxic ion sensitive contrast
agents attached with EPR and MRI visible molecules as labels sensitive to fluorometric and
magnetic resonance techniques respectively. It is attributed that cytochromes are best
candidates as fluorochromic agents while lipopolysaccharides and carbamates with
paramagnetic agents are potential as MRI contrast agents. More likely, fast imaging
techniques such as parallel phase array, SENSE, FISP MRI imaging, xenon based
PARACEST techniques may be available to capture rapid NO changes or distribution
associated with metabolic short lived events such as oxygen sensitivity, sodium sensitivity
in the intact tissue. In future, advances in ionic ratiomatric techniques will be available to
capture the short living NO molecular distribution in cells as real-time events associated
with motion. Still major issues to resolve are:



lack of NO sensitive EPR or MRI contrast agent to generate quicker and enough MRI or
EPR signal visible as image;
the EPR and MRI signals are not true representative of NO concentrations;
the images generated from MRI visible paramagnetic ions or EPR visible spin trap
agents may represent several physical and molecular events sensitive to attached NO
quenching carbamates or polysaccharides for MRI and cytochrome proteins for EPR.
The biggest disadvantage of MRI is its low molecular sensitivity. The need for a high dose of
imaging agents (typically more than 100 mM injection) undoubtedly raises major concerns
Inhibition of Nitric Oxide Synthase Gene Expression:
In vivo Imaging Approaches of Nitric Oxide with Multimodal Imaging
159
about the potential toxicity caused by such studies. Lastly, relatively long data acquisition
time is typically needed to generate a detectable MR signal in MR imaging. Since the lifetime
of NO and/or related species is usually short, the accuracy of MRI measurement of NO
concentration is questionable. After these issues are resolved, it will need sequence of
investigations to calibrate linearity of NO concentration and image signal intensity, image
reproducibility, image component analysis, signal measurement accuracy and precision of
NO sensitive imaging techniques as shown in Table 4.
Target
Modality
Resolution
Sensitivity
Penetration
Clinical potential
NO
Optical
++++
++++
+
Low
NO
EPR
++
++++
+++
Medium
NO
MRI
+++
+
++++
Medium
NO
PET
++
++++
+++
Medium
cNOS
Optical
++++
+++
++
Low
cNOS
PET
+++
++++
++++
Medium-High
Table 4. Comparison of molecular imaging of NO and cNOS expression
5.1 Real-time NO imaging
Nitric oxide (NO) is a small uncharged free radical abundant in nanomolar quantities that is
involved in diverse physiological and pathophysiological mechanisms.

When generated in vascular endothelial cells, NO plays a key role in vascular tone
regulation and EPR/optical/MRI visible signals visualize the vascular physiology
(induced fitting of L-arginine specific NOS active site) as prosposed by author to inhibit
NOS and low NO production shown in Figure 5.
In this direction, short lived NO specific amplifier-coupled fluorescent indicator was
reported to visualize physiological nanomolar dynamics of NO in living cells (detection
limit of 0.1 nM)[Ueno, et al. 2002, Burnett, et al. 2002]. An amplifier-coupled fluorescent
indicator visualizes NO in single living cells. Its domain structures are sGCα, sGCβ, CGY,
sGCα-CGY, and sGCβ-CGY. This genetically encoded high-sensitive indicator revealed that
≈1 nM of NO was enough to relax blood vessels [Ueno, et al. 2002, Burnett, et al. 2002,
Barouch, et al. 2002, Hillebrand et al. 2009, Ciplak, et al. 2009, Gragasin, et al. 2004]. The
nanomolar range of basal endothelial NO thus revealed appeared to be fundamental to
vascular homeostasis. A fuorescent probe, 1,3,5,7-tetramethyl-2,6-dicarbethoxy-8-(3,4diaminophenyl)-diXuoroboradiaza-s-indacence (TMDCDABODIPY), produced real-time
image NO of PC12 cells, Sf9 cells and human vascular endothelial cells in the presence of Larginine with inverted fuorescence microscope [Manuel, et al. 2002]. NO production in the
cells was successfully captured and imaged with improved temporal and spatial resolution.
The bioimaging applications of TMDCDABODIPY as a fuorescent imaging probe for NO in
living cells was compared with the other fuorescent imaging probes for NO. The
TMDCDABODIPY had the advantages of shorter reaction time, higher photostability and
minimal background. The study has indicated that TMDCDABODIPY-based fuorescence
microscopy may be a promising approach for NO-related pathological and physiological
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Enzyme Inhibition and Bioapplications
studies [Hiratsuka, et al. 2009]. A recent review highlighted the possibilities if nitric oxide
bioimaging becomes reality as clinical imaging tool [Hong, et al. 2009]. Current state-of-theart multimodality imaging may detect NO and NOS enzyme expression [Matter et al. 2004,
Zhang, et al. 2011]. Optical (fluorescence, chemiluminescence, and bioluminescence),
electron paramagnetic resonance (EPR), magnetic resonance (MR), and positron emission
tomography (PET) noninvasive imaging techniques may reveal the biodistribution of NO or
Fig. 15. (on top) Formation and release of NO in tissues is shown. No is trapped by
Fe2+(MGD)2 to make MRI/EPR visible trap NO-(MGD)2-Fe3+. (in middle row)The figure
represents how different levels of Nitric oxide (1) activate cytosolic guanylate cyclase (2) and
thus elevate intracellular levels of cyclic GMP (3 and 4) as secondary messenger. The cyclic
GMP stimulates response (5) in different tissues and nitric oxide can be detected as
biomarker of tissue pathology.(at bottom) The relation of released NO across the membrane
is shown with different biological and physiological changes to generate EPR and MRI
signal visible by imaging techniques. Notice the NO regulation by enzyme guanylate cyclase
over intracellular responses (cell injury, inflammation etc.) sensitive to spin trap MRI/EPR
image signals. A proposal of multimodal nitric oxide imaging is shown for development of
multimodal spin trap applicable in imaging.
Inhibition of Nitric Oxide Synthase Gene Expression:
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161
NOS in living subjects with high fidelity which will greatly facilitate scientists and clinicians
in the development of new drugs and patient management[Oliveira, et al. 2010, Payne, et al.
2010]. Lastly, novel NO/NOS imaging agents with optimal in vivo stability and suitable
pharmacokinetics such as pteridines, imidazoles, quinolines for clinical translation will
benefit in patient management [Payne, et al. 2010, Bonnefous, et al. 2009].
Imaging of NOS expression mainly relies on labeling high affinity, preferably also high
specificity, NOS inhibitors with optimal membrane permeability and pharmacokinetics. Of
all the fluorescence imaging agents for NO, the DAFC compounds are the most widely used.
The requirement of oxygen restricts their applicability in cancer-related NO imaging to a
certain extent, although some have been tested for NO imaging during tumor angiogenesis
[Kashiwagi, et al. 2005]. Copper-based fluorescent compounds do not require oxygen for
NO imaging, yet the stability and toxicity of those compounds are questionable. FRET-based
NO imaging is a clever approach, yet it is too technically challenging to be widely available
and useful. With spin-trapping technology, EPR imaging of NO can be quite accurate, yet it
is not applicable for clinical studies due to many reasons. MRI can have high resolution and
large field-of-view for potential clinical imaging studies, yet it is usually quite slow in data
acquisition and the intrinsic low sensitivity makes molecular MRI unlikely to succeed in the
clinic. NOS imaging is also important in elucidating NO-related physiological and
pathological processes. NOS imaging has not been well studied to date. To the best of our
knowledge, no clinical studies of NO or NOS imaging have been reported. Whether
NO/NOS imaging can help and improve patient management remains to be demonstrated
in the future. The deregulation of iNOS in melanoma has been correlated directly with poor
survival [Madunapantula, et al. 2008] and NOS expression in patients with neurological
disorder has been studied [Broholm, et al. 2004]. Given the fact that NO and NOS plays
diverse roles in many diseases such as inflammation, neurodegeneration, cancer, and
vascular malfunctions, further understanding of the disease mechanisms and clarification of
the temporal/ spatial NO/NOS expression are certainly very important in patient
management. Whether imaging NO or NOS is more relevant depends primarily on the
disease. eNOS and nNOS typically only produces low levels of NO; therefore, imaging NO
is expected to be more difficult than imaging the NOSs themselves. On the other hand,
iNOS can produce high concentrations of NO in immunological processes and/or cancer;
hence imaging NO may be more desirable in these scenarios. Granted that NO imaging is
more relevant since it directly affects the physiological and pathological processes, imaging
of NO may not always be feasible and imaging NOS expression (as an alternative) can
provide valuable insights. The notion that nitrite reduction can also serve as a possible
source of biologically relevant NO tilted the balance toward NO imaging [Bryan, 2006].
Nitrite reduction to NO can occur via several routes involving enzymes, proteins, vitamins,
or even simple protons [Galdwin, 2004, Lundberg, et al. 2005]. Nitrite is now under active
investigations in physiology, pathophysiology, and therapeutics. The ideas that nitrite can
be a “storage” form of NO and the possible importance of dietary nitrite are two vibrant
research areas today. Similar to the important role of L-arginine for optimal NOS activity,
nitrite may emerge as an essential nutrient. This pathway may serve as a backup system for
NO generation in conditions such as hypoxia, in which the NOS/L-arginine system is
compromised. We envision that imaging NO and NOS in the same system will undoubtedly
give a more thorough picture in understanding the functions of NO and NOS. For potential
clinical translation of NO/NOS imaging probes, PET tracers have the best chances of
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Enzyme Inhibition and Bioapplications
success. Each imaging modality has its advantages and disadvantages in terms of
sensitivity, spatial resolution, temporal resolution, probe availability, and cost [Massoud, et
al. 2003]. With the development of hybrid imaging systems such as PET/CT, SPECT/CT,
and PET/MRI Catana et al. 2006; Sharma, 2002, 2008, 2011;Townsend, et al. 2002], a
multimodality approach may offer synergistic advantages over any single modality alone in
the future.
6. Conclusion
Nitric oxide (NO) in vivo image visualizes the distribution of NO using the ‘‘spin-trapping’’
technique. Available NO visualizing chemical probes combined with the spin traps and
fluorescent dyes are promising. Physiological mechanisms of NO production by NOS and
role of NOS gene expression altered by NOS inhibitors is illustrated. Scope: Different
multimodal contrast mechanisms are introduced using nitrosyl-iron complexes as MRI-PET
signal intensity enhancers and fluorescent or bioluminescent NO visualizing cheletropic
trap agents. NO synthase enzyme is the source of NO formation in presence of competitive
inhibitor N-monomethyl-L-arginine in the tissues.



General Significance: NO makes stable paramagnetic with N-methyl-D-glucamine
dithiocarbamate (MGD), as (MGD)2-Fe(II)-NO metal complex in vivo by spin trapping
and may serve as effective EPR and/or MRI contrast agent than other less stable
nitrogen containing radicals, such as bound nitric oxides. MRI spin-trapping induces
the magnetic resonance relaxation changes of neighboring protons and visualizes
spatial distributions of NO free radicals in pathologic organs and tissues but not
confirmed. Other paramagnetic NO-Fe-dithiacarbamate metal complexes serve as
contrast agents in EPR bioimaging. Recently, real time physiological in vivo monitoring
using tissue physical properties such as fluorescence and magnetic resonance in tissues
is revisted to visualize nanomolar range of nitric oxide mapping.
Major conclusions: 1. More applications of NO bioimaging and fluorescent probes are
emerging in imaging apoptosis, vascular imaging, smooth muscle cells,
neurodegeneration, calcium channel imaging and endothelial stress. 2. The status of NO
production by NOS in cells gives insight of inflammation, fertilization, mitochondrial
oxidation injury, bone structure and adrenal gland and retina. 3. The real time NO
imaging application is emerging in animals and plants.
Major highlights:

Nitric oxide bioimaging is emerging as rapid noninvasive multimodal molecular
imaging techniques by fluorescent, MRI, PET, optical bioimaging for wider
applications;

Significant progress is made to explain the mechanism of multimodal NO spin trap
contrast agent enhanced MRI and fluorescent bioimaging;

Both MRI and fluorescent contrast agents may serve as potential MRI/fluorescent
multimodal spin traps as well as contrast agents in clinical or bioimaging;

NO sensitive MRI of vital soft tissues is major achievement to visualize very short
lived NO in real time manner.

Development of NOS inhibitors or NO production suppressors and in vivo
NO/NOS evaluation is a booming industry in diagnostics of stroke, sepsis,
neurodegeneration, inflammation and new drug discovery.
Inhibition of Nitric Oxide Synthase Gene Expression:
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163
Spin trap contrast agents may enable the simultaneous EPR-MRI and fluorescent imaging of
NO distribution in the organs and tissues in muscle vascular system, retina, adrenal gland,
brain, bone marrow to monitor intracellular metabolic events such as apoptosis,
inflammation, nerve activation and calcium ions etc. To our knowledge, there are no reports
of successfully imaging or detecting free radicals such as NO in vivo clinical imaging. Due
to short lifetime and rapid diffusion of NO in tissue, it is a challenge to devise any effective
relaxation enhancement mechanism(s) for noninvasive clinical imaging. NMR spin trapping
may serve to overcome these problems of NO short lifetime and diffusion to some extent.
7. Appendix 1: Protocols on NOS and NO* radical detection
Source: Cai H., Dikalov S, Griendling K.K., Harrison, D.G.(2007) Detection of reactive
oxygen species and nitric oxide in vascular cells and tissues. Chapter 20 In: Methods in
Molecular Medicine, Vascular Biology Protocols. Editor: Sreejayan N. and Ron J., Humana
Press Inc. Totowa, NJ. Pp 293-311.
Electronic configuration in NO* with 11 valence electrons:
(K2K2)(2sb)2(2s*)2(2pb)4(2pb)2(2p*)1
Detection of Nitric Oxide Radical
It has been challenging to detect nitric oxide radical (NO•) directly from biological samples.
NO• “production” is measured by NO• synthase activity using the L-arginine conversion
assay or NO• metabolites nitrite and nitrate using the Griess reagent. New methods are
NO•-selective electrode and ESR represent specific and quantitative assays for detection of
functional NO•.
Materials needed:
1.
2.
3.
NO•-Specific Microelectrode
Nafion and o-PD coated carbon electrode can directly detect NO• in the low
micromolar range.
ESR with NO•-Specific Spin Traps
Dithiocarbamate (DTC), N-methylglucamine dithiocarbamate (MGD) detect
extracellular NO*, and diethyldithiocarbamate (DETC) trap and detect the NO* in
cellular lipid membrane.
iron-MGD and iron-DETC.
ESR measurement:

Endothelial or vascular smooth muscle cells.

Modified Krebs/HEPES buffer (see Subheading 2.1.1.2.).

Cyclic
hydroxylamine
CPH
or
1-hydroxy-3-methoxycarbonyl-2,2,5,5tetramethylpyrrolidine

(CMH) stock solution (10 mM) (Alexis Biochemicals, San Diego, CA,

USA) in modified Kreb’s/HEPES buffer containing metal chelator, 25–50mM

deferoximine, and 3 5mM DETC. This stock solution should be de-oxygenated by

nitrogen gas continuously to maintain low background oxidation of the spin traps.

Lysis buffer containing protease inhibitors: 50mM Tris–HCl buffer, pH 7.4,

containing 0.1mM ethylenediamine tetraacetic acid (EDTA), 0.1mM ethylene
164
Enzyme Inhibition and Bioapplications

glycol tetraacetic acid (EGTA), 1mM phenylmethyl sulfonyl fluoride, 2mM
bestatin, 1mM pepstatin, and 2mM leupeptin.

NADPH (0.2 mM).

Xanthine (0.1 mM).
Detection of Nitric Oxide Radical
NO•-Specific Microelectrode







Carbon fiber electrodes (100µm length and 30µm outer diameter; Word Precision
Instruments, Sarasota, FL, USA).
o-PD solution (in 0.1M PBS with 100µM ascorbic acid).
Nafion (5% in aliphatic alcohols; Sigma-Aldrich).
Modified Kreb’s/HEPES buffer.
Axopatch 200B amplifier (Axon Instruments, Union City, CA, USA).
Silver/silver chloride reference electrode.
Iron-DETC for Trapping of NO•







Culture endothelial cells.
Saline (0.9% NaCl).
Fe2+(DETC)2 : FeSO4·7H2O, 4.45 mg/10 ml for 1.6 mmol/l stock and DETC, 7.21 mg/10
ml for 3.2 mmol/l stock.
PBS.
Modified Kreb’s/HEPES buffer.
Ferrous sulfate (4 mM).
N-methyl-d-glucamine dithiocarbamate MGD (20 mM).
Methods:
Detection of Nitric Oxide Radical
NO•-Specific Microelectrode






Coat bare carbon fiber electrodes (100µm length and 30µm outer diameter; Word
Precision Instruments) with nafion and o-PD. Coat with freshly made o-PD solution at
constant potential (+0.9V vs. Ag/AgCl reference electrode) for 45 min. Dip in nafion
solution for 3 s and dry for 5 min at 85°C. The nafion-coating cycle should be repeated
10–15 times.
Culture endothelial cells on 35-mm dishes or prepare fresh tissue samples in freshly
made modified Kreb’s/HEPES buffer.
Place the electrode tip at the surface of an individual cell, endocardium, or lumen of
blood vessels, and then withdraw precisely 5µm.
Record NO•-dependent oxidation currents (voltage clamp mode, hold at 0.65 V,
approximately the voltage for peak NO• oxidation) immediately after addition of
agonists using an Axopatch 200B amplifier (Axon Instruments). A silver/silver chloride
reference electrode is used. Use pCLAMP 7.0 program (Axon Instruments) for delivery
of voltage protocols and data acquisition and analysis.
Calculate NO• concentrations from a standard curve obtained using dilutions of deoxygenated, saturated NO* gas solutions.
Inhibition of Nitric Oxide Synthase Gene Expression:
In vivo Imaging Approaches of Nitric Oxide with Multimodal Imaging
165
Iron-DETC Protocol for Intracellular Trapping of NO•







Culture endothelial cells on 100-mm Petri dishes or prepare vascular segments (6–12, 2mm vessel segments).
Bubble freshly prepared saline (0.9% NaCl) with nitrogen gas to remove oxygen.
Aspirate media and rinse cells with warm PBS once, add 1.5 ml modified
Kreb’s/HEPES buffer with or without desired agonists, then mix Fe2+(DETC)2, and
immediately add to culture dish (500µl of each solution, final volume 2.0 ml).
Incubate in cell culture incubator for desired period for cumulative trapping of NO•.
Aspirate buffer, gently collect cells into a 1-ml insulin syringe, snap freeze in liquid
nitrogen, then transfer sample column into a finger dewer, and capture Fe2+(DETC)2–
NO• signal using ESR at the following settings:
Bruker EMX: Field sweep, 160 G; microwave frequency, 9.39 GHz; microwave power,
10MW; modulation amplitude, 3 G; conversion time, 2621 ms; time constant, 328 ms;
modulation amplitude, 3 G; receiver gain, 1×104; and four scans.
Miniscope 200: Biofield, 3267; field sweep, 100 G; microwave frequency, 9.78 GHz;
microwave power, 40 mW; modulation amplitude, 10 G; 4096 points resolution; and
receiver gain, 900.
Iron-MGD Protocol for Extracellular Trapping of NO•






Culture endothelial cells on 100-mm Petri dishes or prepare vascular segments (6–12, 2mm vessel segments).
Bubble freshly prepared saline (0.9% NaCl) with nitrogen gas to remove oxygen, and
then make stock solutions of FeSO4·7H2O, 4 mM, and MGD, 20 mM.
Prepare stock solutions of Fe2+MGD by mixing FeSO4 and MGD at the ratio of 1:5 or
1:10 (final Fe concentration: 0.5 mM).
Aspirate media and rinse cells with warm PBS once, add 1.5 ml modified
Kreb’s/HEPES buffer with or without desired agonists, then mix Fe2+(MGD)2, and
immediately add to culture dish.
Incubate in cell culture incubator for desired period for cumulative trapping of NO•.
Collect 1 ml of post-incubation supernatant into a 1-ml insulin syringe and snap freeze
in liquid nitrogen, then transfer sample column into a finger dewer, and capture
Fe2+MGD–NO• signal using ESR settings as described above for Fe2+(DETC)2.
8. Acknowledgements
Author acknowledge the manuscript preparation, corrections, expert suggestions by Dr
Soonjo Kwon, Utah State University, Logan, UT and Professor Ching J Chen at Florida State
University, Tallahassee for extending expert modifications and comments in this
manuscript. Prof Avdhesh Sharma, JNV University, corrected manuscript.
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6
Transcriptional Bursting
in the Tryptophan Operon of E. coli and Its
Effect on the System Stochastic Dynamics
Emanuel Salazar-Cavazos and Moisés Santillán
Centro de Investigación y de Estudios Avanzados del IPN, Unidad Monterrey,
Parque de Investigación e Innovación Tecnológica, Apodaca NL
México
1. Introduction
Transcriptional bursting, also known as transcriptional pulsing, is a fundamental property
of genes from bacteria to humans (Chubb et al., 2006; Golding et al., 2005; Raj et al., 2006).
Transcription of genes, the process which transforms the stable code written in DNA into the
mobile RNA message can occur in "bursts" or "pulses". This phenomenon has recently come
to light with the advent of new technologies, such as MS2 tagging, to detect RNA production
in single cells, allowing precise measurements of RNA number, or RNA appearance at
the gene. Other, more widespread techniques, such as Northern Blotting, Microarrays,
RT-PCR and RNA-Seq, measure bulk RNA levels from homogeneous population extracts.
These techniques lose dynamic information from individual cells, and give the impression
transcription is a continuous smooth process. The reality is that transcription is irregular, with
strong periods of activity, interspersed by long periods of inactivity. Averaged over millions of
cells, this appears continuous. But at the individual cell level, there is considerable variability,
and for most genes, very little activity at any one time.
The bursting phenomenon, as opposed to simple probabilistic models of transcription, can
account for the high variability in gene expression occurring between cells in isogenic
populations (Blake et al., 2003). This variability in turn can have tremendous consequences
on cell behaviour, and must be mitigated or integrated. In certain contexts, such as the
survival of microbes in rapidly changing stressful environments, or several types of scattered
differentiation, the variability may be essential (Losick & Desplan, 2008). Variability also
impacts upon the effectiveness of clinical treatment, with resistance of bacteria to antibiotics
demonstrably caused by non-genetic differences (Lewis, 2010). Variability in gene expression
may also contribute to resistance of sub-populations of cancer cells to chemotherapy (Sharma
et al., 2010).
Bursting may result from the stochastic nature of biochemical events superimposed upon a
2 or more step fluctuation. In its most simple form, the gene can exist in 2 states, one where
activity is negligible and one where there is a certain probability of activation (Raj & van
Oudenaarden, 2008). Only in the second state does transcription readily occur. Whilst the
nuclear and signalling landscapes of complex eukaryotic nuclei are likely to favour more
than two simple states—for example, there are over twenty post-translational modifications
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Enzyme Inhibition and Bioapplications
Will-be-set-by-IN-TECH
of nucleosomes known, this simple two step model provides a reasonable framework for
understanding the changing probabilities affecting transcription. What do the restrictive
and permissive states represent? An attractive idea is that the repressed state is a closed
chromatin conformation whilst the permissive state is an open one. Another hypothesis is
that the fluctuations reflect transition between bound pre-initiation complexes (permissive)
and dissociated ones (restrictive) (Blake et al., 2003; Ross et al., 1994). Bursts may also
result from bursty signalling, cell cycle effects or movement of chromatin to and from
transcription factories. Nonetheless, to the best of our knowledge, there is no generally
accepted explanation for this phenomenon. Transcriptional bursting in prokaryotic cell is
particularly puzzling given the simplicity of the transcription initiation process, as opposed
to eukaryotic cells.
There is evidence that the cooperative interaction of distant operators through a single
repressor molecule has a strong influence on the transcriptional bursting observed in the lac
operon of E. coli (Choi et al., 2008). Since the repression regulatory mechanism in E. coli’s
trp operon involves cooperativity between two repressor molecules bound to neighbouring
operators, it is interesting to investigate whether such cooperative interaction has any effect
upon the system transcriptional dynamics. The present chapter is advocated to tackling
this question from a mathematical modelling perspective. We also investigate the effects
of the enzymatic feedback-inhibition regulatory mechanism (also present on the trp operon
regulatory pathway) on the system dynamic behaviour.
2. Theory
2.1 The trp operon
The amino acid tryptophan can be synthesized by bacteria like E. coli through a series of
catalysed reactions. The catalysing enzymes in E. coli are made up of the polypeptides
encoded by the tryptophan operon genes: trpE, trpD, trpC, trpB, and trpA, which are
transcribed from trpE to trpA. Transcription is initiated at promoter trpP, which is located
upstream from gene trpE. The trp operon is regulated by three different negative-feedback
mechanisms: repression, transcription attenuation, and enzyme inhibition. Below these
regulatory mechanisms are briefly reviewed based on (Brown et al., 1999; Grillo et al., 1999;
Jeeves et al., 1999; Xie et al., 2003; Yanofsky, 2000; Yanofsky & Crawford, 1987). The reader
should consult Figure 1 for a better understanding.
Repression in the trp operon is mediated by three operators (O1, O2, and O3) overlapping
with the operon promoter, trpP (see Figure 1A). When an active repressor is bound to an
operator it blocks the binding of a RNA polymerase to trpP and prevents transcription
initiation. The trp repressor normally exists as a dimeric protein (called the trp aporepressor)
and may or may not be complexed with tryptophan (Trp). Each portion of the trp aporepressor
has a binding site for tryptophan.
The trp aporepressor cannot bind the operators tightly when not complexed with tryptophan.
However, if two tryptophan molecules bind to their respective binding sites the trp
aporepressor is converted into a functional repressor. The resulting functional repressor
complex can tightly bind to the trp operators and so the synthesis of the tryptophan
producing enzymes is prevented. This sequence constitutes the repression negative-feedback
mechanism: an increase in the concentration of tryptophan induces an increase in the
concentration of the functional repressor, thus preventing the synthesis of tryptophan.
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
A
1813
B
mRNA hairpin 1:2
I
R
T
P
II
tRNA
mRNA hairpin 2:3
trpP
O3
O2
O1
trpL
trpE
C
III
Anthranilate
Synthase
Ribosome
mRNA hairpin 3:4
Tryptophan
Biosynthesis
T
TrpE
TrpD
TrpC
TrpB
TrpA
trpP
trpP
trpE
trpD
trpC
trpB
trpL
trpE
trpA
D
DNA, structural gene
RNA Polymerase
DNA, promoter region
mRNA
Trp tRNA
Trp molecules
Repressor molecule
Trp polypeptides
Ribosome
Anthranilate synthase
Fig. 1. Schematic representation of the three regulatory mechanisms found in the tryptophan
operon: A) repression, B) transcription attenuation, and C) enzyme inhibition. A glossary
with the meaning of all the geometric forms in this figure is shown in panel D. See the main
text for further explanation.
Transcription attenuation works by promoting an early termination of mRNA transcription,
see Figure 1B. The transcription starting site in the trp operon is separated from trpE by a
leader region responsible for attenuation control. The transcript of this leader region consists
of four segments (termed Segments 1, 2, 3, and 4) which can form three stable hairpin
structures between consecutive segments. After the first two segments are transcribed they
form a hairpin which stops transcription (c.f. Figure 1B-I). When a ribosome binds the
nascent mRNA, it disrupts Hairpin 1:2 and transcription is re-initiated along with translation.
Segment 1 contains two tryptophan codons in tandem. If there is scarcity of tryptophan,
and thus of loaded tRNATrp , the ribosome stalls in the first segment. The development
of Hairpin 2:3 (the antiterminator) is then facilitated, and transcription proceeds until the
end (c.f. Figure 1B-II). However, if tryptophan is abundant, the ribosome rapidly finishes
translation of Segments 1 and 2 and promotes the formation of a stable hairpin structure
between Segments 3 and 4 (c.f. Figure 1B-III). RNA polymerase molecules recognize this
hairpin structure as a termination signal and transcription is prematurely terminated.
Enzyme inhibition takes place through anthranilate synthase, the first enzyme to catalyse a
reaction in the catalytic pathway that leads to the synthesis of tryptophan from chorismate.
This enzyme is a hetero-tetramer consisting of two TrpE and two TrpD polypeptides.
Anthranilate synthase is inhibited by tryptophan through negative feedback. This feedback
inhibition is achieved when the TrpE subunits in anthranilate synthase are individually bound
by a tryptophan molecule, see Figure 1C. Therefore, an excess of intracellular tryptophan
inactivates most of the anthranilate synthase protein avoiding the production of more
tryptophan.
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Will-be-set-by-IN-TECH
3. Methods
3.1 Model development
There are three different repressor binding sites (operators) overlapping the trp promoter.
Hence, the promoter can be in eight different states, with each operator being either free or
bound by a repressor molecule. Furthermore, when two repressor molecules are bound to
the first and second operators, they do it cooperatively (Grillo et al., 1999). As discussed in
Appendix A, this cooperativity allows the grouping of the promoter states into two different
sets that we term the permissive and the restrictive global states. The transitions within each
global state and those from the permissive global state to the restrictive global state being
much faster than those from the restrictive global to the permissive global states. This fact
justifies the assumption that the system “instantaneously” reaches a stationary probability
distribution for the states within every global state. This supposition in turn permits the
derivation of the following expressions for the transition rates from the permissive into the
restrictive global states (k+ ), and vice versa (k− ):
k+ =
k− =
+
k+
i R A /K j + k j R A /Ki
1 + R A /Ki + R A /K j
−
k−
i + kj
kc
,
,
(1)
(2)
+
where k+
i and k j are the rates for the reactions where a repressor molecule binds to the
−
first and second operators, respectively; k−
i and k j are the rates for the reactions in which
a repressor molecule detaches from the first and second operator; R A is the number of active
+
− +
repressors; Ki = k−
i /k i ; and K j = k j /k j .
On the other hand, when the promoter is in the permissive global state, the probability that it is
not bound by any repressor and so it is free to be bound by a polymerase to start transcription
is
1
PR =
.
(3)
(1 + R A /Kk )(1 + R A /Ki + R A /K j )
Repressor molecules are activated when they are bound by a couple of tryptophan molecules.
The kinetics of repressor activation were analysed in (Santillán & Zeron, 2004), where the
number of active repressors is demonstrated to be given by
R A = RT
T
T + KT
2
,
(4)
where R T stands for the total number of repressor molecules, while T is the tryptophan
molecule count. Substitution of Eqn. (4) into Eqns. (1) and (3) permits the calculation of
the promoter inactivation rate (k+ ) and the probability PR in terms of the tryptophan level
(T).
Due to transcriptional attenuation, only a fraction of the polymerase molecules that initiate
transcription reach the end of the trp genes and produce functional mRNA molecules, which
in turn are translated to produce the proteins coded by the trp genes. Santillán and Zeron
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
1835
(2004) found that the probability that transcription is not prematurely terminated due to
transcriptional attenuation is:
1 + 2αT
PA =
,
(5)
(1 + αT )2
with α a parameter to be estimated.
It follows from the considerations in the previous paragraphs that a promoter in the restrictive
global state is completely incapable of being expressed, but its activity level when it is in the
permissive global state is a function of the tryptophan level T and is given by the product of
PR and PA . Therefore, if k E denotes the rate of enzyme synthesis by a fully active promoter,
the enzyme synthesis rate when the promoter is in the permissive state at a given tryptophan
level turns out to be:
k E PR ( T ) PA ( T ).
(6)
Tryptophan is synthesized by proteins which are assembled from the polypeptides coded by
the trp genes. Conversely, tryptophan is mainly consumed in the synthesis of all kinds of
proteins in E. coli. Thus, the equation governing the tryptophan-level dynamics is:
dT
= k T EPI ( T ) − ρ( T ) − μT,
dt
where μ is the bacterial growth rate, k T is the tryptophan rate of synthesis per enzyme
molecule
Kn
PI ( T ) = n I n
T + KI
is the probability that an enzyme molecule is not feedback inhibited by tryptophan, and
ρ( T ) = ρmax
T
T + Kρ
is the rate of tryptophan consumption associated to protein synthesis. If we assume that these
processes are much faster than those associated to gene expression and protein degradation,
then we can make the following quasi-steady state approximation: dT/dt = 0, and the
tryptophan molecule count can be uniquely calculated in terms of the enzyme molecule count
as the root of the following algebraic equation:
k T EPI ( T ) − ρ( T ) − μT = 0.
(7)
Following previous modelling studies we assume that the enzyme degradation rate is
negligible as compared with the bacterial growth rate, μ. On the other hand, instead of
considering a cell that grows exponentially, we assume that we have a constant-volume cell
and that the effective enzyme degradation rate is μ.
The facts previously discussed in the present section provide enough information to develop
a model for the trp operon regulatory pathway. This model consists of four chemical reactions:
promoter activation, promoter inactivation, enzyme synthesis, and enzyme degradation,
whose rates are k− , k+ ( T ( E)) R A ( T ( E)), k E PR ( T ( E)) PA ( T ( E)), and μE, respectively. Figure
2 provides a schematic representation of such a model. It is worth emphasizing that
the repression regulatory feedback loop is implicitly accounted for by functions k+ ( T ( E))
and PR ( T ( E)), that function PA ( T ( E)) corresponds to the attenuation feedback regulatory
mechanism, and the feedback enzyme inhibition is implicit in the function T ( E), obtained
after solving Eqn. (7).
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Enzyme Inhibition and Bioapplications
Will-be-set-by-IN-TECH
Inactive
Promoter
Tryptofano (T )
k-
k+(T (E ))RA(T (E ))
kE PR(T (E )) PA(T (E ))
Enzymes (E)
Active
Promoter
γE
∅
Fig. 2. Schematic representation of the mathematical model here developed for tryptophan
operon gene regulatory circuit.
3.2 Parameter estimation
We paid special attention to the estimation of all the model parameters from reported
experimental data. The parameter values we employ in the present work and the detailed
procedure to estimate them are presented in Appendix B.
3.3 Numerical methods
The time evolution of the reaction network that models the tryptophan operon regulatory
pathway was simulated by means of the Gillespie algorithm, which we implemented in
Python.
4. Results
We carried out stochastic simulations with the model described in the previous section. As
formerly stated, we made use of Gillespie’s algorithm to mimic the system dynamic evolution
for 200,000 min. In the first simulation we employed the parameter values estimated in
Appendix B, which correspond to a wild-type bacterial strain. The results are summarized in
Figure 3. In Figure 3A the cumulative sum of the promoter activity is plotted vs. time. We can
appreciate there the existence of alternated activity and inactivity periods, just like it has been
observed in transcriptional bursting. To further investigate this phenomenon we calculated
the histograms of the permissive and restrictive period lengths. The results are shown in
Figures 3B and 3C, respectively. Observe that both histograms are well fitted by exponential
distributions, in agreement with the reported experimental data on transcriptional bursting.
Finally, we present in Figures 3D and 3E the histograms for the enzyme and tryptophan
molecule counts, respectively.
One feature worth noticing is that the histogram for the enzyme abundance is well fitted by
a gamma distribution with parameters k = 32.5 and θ = 63. This last fact is in agreement
with the existence on transcriptional bursting in the trp operon of E. coli. It has been proved
that in such a case, the protein count obeys a gamma distribution, with parameters k and θ
respectively interpreted as the average number of transcriptional bursts occurring during an
average protein lifetime and the mean number of proteins produced per burst (Shahrezaei &
Swain, 2008). It is also interesting to point out that the coefficient of variation in the tryptophan
molecule count is similar to that of the enzyme molecule count.
Activity Cumulative Sum
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
3.5
1857
A
3
2.5
2
1.5
1
0
100
200
300
400
B
0.4
μ = 0.35 min
0.3
0.2
0.1
0
0.5
1
1.5
2
2.5
Normalized Frequency
Normalized Frequency
Time (min)
0.5
0.3
C
0.25
0.2
μ = 8.88 min
0.15
0.1
0.05
0
3
0
0.12
D
μ = 1988
CV = 0.19
0.1
0.08
0.06
0.04
0.02
0
0
500
1000
1500
2000
2500
Enzyme Count
20
40
60
80
Inactive Interval (min)
3000
Normalized Frequency
Normalized Frequency
Active Interval (min)
0.12
E
μ = 8.2e4
CV = 0.16
0.1
0.08
0.06
0.04
0.02
0
0
2e4
4e4
6e4
8e4
10e4
12e4
Tryptophan Count
Fig. 3. Statistical analysis of the simulation corresponding to the wild-type strain. A) Plot of
the cumulative promoter activity vs. time. B) Histogram of the promoter permissive time
intervals and best fit to an exponential distribution. C) Histogram of the promoter restrictive
time intervals and best fit to an exponential distribution. D) Histogram of the enzyme
molecule count and best fit to a gamma distribution. E) Histogram of the tryptophan
molecule count.
In order to understand the influence of the slow promoter gating between the global
permissive and restrictive states on the operon dynamics, we increased the value of
−
parameters k+
x and k x (x = I, j, k) by a factor of 100. In this way, the promoter switching rate
among all its available states gets faster, without altering each state’s stationary probability.
We repeated the simulation described in the previous paragraph with this new parameter set
and the results are condensed in Figure 4.
We observe by comparing Figures 3A and 4A that there are many more alternated activity
and inactivity periods in the fast-switching model than in that corresponding to the wild
type strain. Concomitantly, the periods are shorter in the former case. Interestingly, the
accumulated promoter activity is quite similar for both models. This last result comes from
−
our increasing parameters k+
x and k x by the same factor, and is in agreement with the fact that
the mean enzyme and tryptophan counts are quite similar in both models (see below).
In Figures 4B and 4C we present the histograms for the activity and inactivity periods, and
the corresponding fits to exponential distributions. By comparing with the wild-type period
distributions we can see that the mean values of both the activity and inactivity periods
decrease. However the decrease of the activity-period average is about twice as large as that
of the average of the inactivity period.
Finally, the histograms for the enzyme and tryptophan molecule counts are plotted in Figures
4D and 4E. Notice that the histogram for the enzyme count is well approximated by a gamma
distributions with parameters k = 2079 and θ = 1. Therefore, by making the promoter
switching rate faster we increased the frequency of bursting, but decreased in the same
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Enzyme Inhibition and Bioapplications
Will-be-set-by-IN-TECH
A
3.5
3
2.5
2
1.5
1
0
100
200
300
400
B
0.8
μ = 0.063 min
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
Normalized Frequency
Normalized Frequency
Time (min)
0.4
C
0.3
μ = 4.37 min
0.2
0.1
0
1
0
10
0.3
0.25
D
μ = 2079
CV = 0.022
0.2
0.15
0.1
0.05
0
1900
1950
2000
2050
2100
2150
Enzyme Count
20
30
40
Inactive Interval (min)
2200
2250
Normalized Frequency
Normalized Frequency
Active Interval (min)
0.16
0.14
0.12
μ = 8.5e4
CV = 0.018
E
0.1
0.08
0.06
0.04
0.02
0
8e4
82000 84000 86000 88000 90000
Tryptophan Count
Fig. 4. Statistical analysis of the simulation corresponding to the strain with fast
promoter-transition rates. A) Plot of the cumulative promoter activity vs. time. B) Histogram
of the promoter permissive time intervals and best fit to an exponential distribution. C)
Histogram of the promoter restrictive time intervals and best fit to an exponential
distribution. D) Histogram of the enzyme molecule count and best fit to a gamma
distribution. E) Histogram of the tryptophan molecule count.
amount the number of proteins synthesized per burst. In that way, the average enzyme
count remains the same (compare with Figure 3D). However, the variation coefficient is much
smaller in the fast promoter switching model than in the wild-type one. Recall
√ that, in the
gamma distribution, the mean and the standard deviation
are:
μ
=
θk
and
σ
=
θ
k, while the
√
variation coefficient is given by CV = σ/μ = 1/ k.
We further simulated a bacterial mutant strain lacking the feedback inhibition regulatory
mechanism. To mimic this mutation we increased the value of parameter K I by two orders of
magnitude, up to K I = 500, 000 molecules. We analysed this last simulation in a similar way
than the previous ones and present the results in Figure 5.
A comparison of Figures 3A and 5A reveals that the promoter level of activation is
generally smaller in the inhibition-less mutant strain than in the wild-type strain, because the
accumulated activity is about three times smaller in the former case. Nonetheless the length
of the activity and inactivity periods seem to be similar. This last assertion is corroborated
by the plots in Figures 5B and 5C, where we can see the the activity and inactivity period
histograms are well fitted by exponential distributions, and that the corresponding mean
values are similar to the corresponding ones in the wild-type strain.
In agreement with the fact that the promoter level of activity is smaller in the inhibition-less
than in the wild-type strain, the mean protein count is smaller in the first case (compare
Figures 3D and 5D). On the other hand, the coefficient of variation (CV) is similar in both cases.
To understand why this happens when one would expect a larger CV in the inhibition-less
Activity Cumulative Sum
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
1879
1.07
A
1.06
1.05
1.04
1.03
1.02
1.01
1
0
100
200
300
400
Normalized Frequency
Normalized Frequency
Time (min)
0.6
B
0.5
0.4
μ = 0.30 min
0.3
0.2
0.1
0
0.5
1
1.5
2
C
0.4
μ = 6.37 min
0.3
0.2
0.1
0
2.5
0
10
D
μ = 46
CV = 0.15
0.3
0.2
0.1
0
10
20
30
40
50
60
20
30
40
50
Inactive Interval (min)
0.4
Normalized Frequency
Normalized Frequency
Active Interval (min)
0.25
0.2
E
μ = 3.4e5
CV = 2.92
0.15
0.1
0.05
0
70
1e5
Enzyme Count
2e5
3e5
4e5
5e5
6e5
Tryptophan Count
Normalized Enz. Count
Fig. 5. Statistical analysis of the simulation corresponding to the inhibition-less strain. A) Plot
of the cumulative promoter activity vs. time. B) Histogram of the promoter permissive time
intervals and best fit to an exponential distribution. C) Histogram of the promoter restrictive
time intervals and best fit to an exponential distribution. D) Histogram of the enzyme
molecule count and best fit to a gamma distribution. E) Histogram of the tryptophan
molecule count.
1
0.8
0.6
0.4
0.2
0
0
50
100
150
200
150
200
Normalized Trp Count
Time (min)
1
0.8
0.6
0.4
0.2
0
0
50
100
Time (min)
Fig. 6. Plots of the normalized enzyme count and the normalized tryptophan count,
averaged over 1,000 independent simulations, vs. time for the wild-type (blue line), the fast
promoter-transition (red line), and the inhibition-less (green line) E. coli strains.
mutant because of the reduced enzyme count, we fitted the histogram in Figure 5D and found
that the best fit is obtained with parameters k = 40 and θ = 1.2. Since the values of k for the
inhibition-less and the wild-type strains are similar, we conclude that the burst frequency is
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Will-be-set-by-IN-TECH
comparable in both cases. However, the number of proteins produced per burst is notably
smaller in the inhibition-less strain due to the reduced promoter activity, as well as to the
increased level of transcriptional attenuation.
We note that, in contrast with the two previous simulations, the variation coefficient of the
tryptophan molecular count in the inhibition-less strain is much larger than the coefficient of
variation for the enzyme count. To our understanding, this happens because, being enzyme
inhibition absent, there is a highly non-linear relation between the enzyme and tryptophan
molecule counts.
In order to investigate the effect of the inhibition-less mutation and of the increased promoter
switching rate on the trp operon response time, we carried out 1,000 simulations with each
bacterial strain (including the wild type), starting with the promoter in the restrictive state
and zero enzyme and tryptophan molecules as initial conditions. Then, we averaged the
enzyme and the tryptophan counts over all the simulations, and plotted the results in Figure
6 to compare how fast each strain approaches the steady state. We can see there that the
inhibition-less strain is the one with the shortest response time, followed by the wild type and
the fast promoter switching strains, respectively.
5. Concluding remarks
In this work we have introduced a stochastic mathematical model for the tryptophan operon.
Our objectives were twofold: 1) to investigate whether the reported reaction rates of the
interaction between repressor and the operators can give rise to transcriptional bursting; and
2) to study the dynamic effects of transcriptional bursting, if it exists, and of the feedback
enzyme-inhibition regulatory mechanism.
Regarding the first objective our results indicate that, indeed, the reported reaction rates
make the promoter switching between its available states slow enough so as to give
rise to transcriptional bursting. As previously discussed, this assertion is supported by
the agreement between our model results and a number of reported experimental facts.
Interestingly, experiments on the lac operon also suggest that transcriptional bursting has its
origin in the kinetics of the repressor-operator interaction. As a matter of fact, Choi et al. (2008)
demonstrated that the cooperativity present when a single repressor molecule is bound to two
distant operators is responsible of generating two different types of bursts in the expression
of lac operon.
In our model we have assumed that the trp promoter activation rate k− is independent of the
tryptophan concentration. This assumption is supported by direct and indirect experimental
measurements of the promoter activation and deactivation rates on the lac and several other
promoters (Choi et al., 2008; So et al., 2011). Those reports demonstrate that modulation of
gene expression is mainly achieved by changing the promoter deactivation rate.
Regarding our second objective, our results allow us to put forward the following conclusions:
• Transcriptional bursting increases the noise level and decreases the system response time
after a nutritional shift. To the best of our understanding, the noise level is increased
because the promoter-transition events become less frequent as the promoter-repressor
interactions slow down, thus enhancing the concomitant stochastic effects. On the
other hand, the faster system response can be explained by a single burst of intense
transcriptional activity, occurring during the first couple of minutes after the nutritional
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
189
11
shift. This burst allows most of the cells to reach tryptophan levels superior to two thirds
of the steady-state level.
• Enzyme inhibition also has important dynamic effects. It increases the noise level
in the enzyme count, but decreases the noise level in the number of tryptophan
molecules. Furthermore, this regulatory mechanism also increases the system response
time. Knowing that the presence of a strong negative feedback is capable of reducing
the noise in a biological system (Austin et al., 2006; Becskei & Serrano, 2000; Dublanche
et al., 2006), we can explain the above observations as follows: the fact that the wild-type
E. coli strain has lower tryptophan levels than the inhibition-free strain means that the
transcriptional-attenuation and the repression feedback loops are weaker in the first strain;
this weakening of both negative feedback loops is responsible for the increment of the noise
level in the enzyme count. For the same reasons, the presence of the enzyme-inhibition
feedback loop reduces the noise level in the tryptophan count, but makes it necessary to
produce much more enzymes to fulfil the required tryptophan production. This increased
enzyme synthesis requirement lengthens the system response time.
Finally, if we assume that having a tryptophan operon with short response times and low noise
levels in the tryptophan molecular count are beneficial traits for E. coli then we can speculate
from the previously discussed facts that evolution has bestowed this system with an optimal
trade-off between short response times and low tryptophan noise.
6. Acknowledgements
This research was partially supported by Consejo Nacional de Ciencia y Tecnología
(CONACyT, MEXICO) under Grant: 55228.
7. Appendix
A. Promoter dynamics modelling
Three different operators are overlap with the trp promoter and each of them can be bound by
a repressor molecule. Therefore, the eight promoter states can be denoted as {i, j, k}, where
i, j, k = 0, 1 represent the binding state of the first, second, and third repressors, respectively.
A zero (one) value means that the corresponding operator is free from (bound by) a repressor
molecule.
+
+
Let k+
i , k j , and k k respectively denote the rates of binding of a repressor molecule to the
−
−
first, second, and third operators, when the other two are free. Similarly, let k−
i , k j , and k k
respectively represent the dissociation rate of a repressor molecule solely bound to the first,
second, and third operators.
It is known that the first and second operators are bound by repressor molecules cooperatively
and the cooperativity constant k c > 1 has been measured. Here we assume that this
cooperativity means that the rate of dissociation for a repressor molecule bound to either the
−
first or the second operator is respectively given by k−
i /k c or k j /k c , when both operators
are bound by repressor molecules. Under this assumption, the eight different promoter
states can be grouped into two sets that we call the permissive and the restrictive global
states. The permissive global state consists of states (0, 0, 0), (1, 0, 0), (0, 1, 0), (0, 0, 1), (1, 0, 1),
and (0, 1, 1), while the restrictive global state consists of (1, 1, 0), and (1, 1, 1). From the
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Will-be-set-by-IN-TECH
way the were constructed, the transitions within each global state and the transitions from
the permissive to the restrictive global state are much faster than the transitions from the
restrictive to the permissive global state. From the above considerations we make an adiabatic
approximations for all the transitions within the permissive state, and thus:
P(0, 0, 0) RKAi = P(1, 0, 0),
P(0, 0, 0) RKAj = P(0, 1, 0),
P(0, 0, 0) RKAk = P(0, 0, 1),
P(0, 0, 1) RKAi = P(1, 0, 1),
P(0, 0, 1) RKAj = P(0, 1, 1),
where P(i, j, k) stands for the probability of state i, j, k within the restrictive global state, R A
+
denotes the number of active repressors, and K x = k−
x /k x (x = i, j, k). It follow from the
equations above and the constraint P(0, 0, 0) + P(1, 0, 0) + P(0, 1, 0) + P(0, 0, 1) + P(1, 0, 1) +
P(0, 1, 1) = 1 that
P(0, 0, 0) = (1+ R /K )(1+1R /K + R /K ) ,
A
k
A
i
A
j
i
P(1, 0, 0) = (1+ R /K )(R1+A /K
,
R A /Ki + R A /K j )
A
k
R /K
P(0, 1, 0) = (1+ R /K )(1+A R j/K + R /K ) ,
A
A
A
i
j
k
A /Kk
,
P(0, 0, 1) = (1+ R /K )(R1+
R A /Ki + R A /K j )
A
k
R A /Ki × R A /Kk
P(1, 0, 1) = (1+ R /K
,
A
k )(1+ R A /Ki + R A /K j )
R /K × R /K
A
j
k
P(0, 1, 1) = (1+ R /KA)(1+
.
R A /Ki + R A /K j )
A
k
Let k+ (k− ) be the transition rate from each of the states in the permissive (restrictive) global
state to each if the states in the restrictive (permissive) global state. From Zeron and Santillán
(2010), these rates are given by
+
k+ = k+
i ( P (0, 1, 0) + P (0, 1, 1)) + k j ( P (1, 0, 0) + P (1, 0, 1)) =
and
k− =
−
k−
i + kj
kc
( P(1, 1, 0) + P(1, 1, 1)) =
+
k+
i R A /K j + k j R A /Ki
1 + R A /Ki + R A /K j
−
k−
i + kj
kc
,
,
since P(1, 1, 0) + P(1, 1, 1) = 1.
Finally, when the promoter is in the permissive global state, the probability that it is not bound
by any repressor and so it is free to be bound by a polymerase to start transcription is
PR = P(0, 0, 0) =
1
.
(1 + R A /Kk )(1 + R A /Ki + R A /K j )
Thus PR can be interpreted as the operator activity level.
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
191
13
B. Estimation of the model parameters
In this work we consider a bacterial doubling time of 40 min, and thus
μ 0.017 min−1 .
From the website E. coli Statistics, the total number of proteins in E. coli is about 3.6
millions, while the average protein size is 360 residues. On the other hand, we have from the
website B1ONUMB3RS (http://bionumbers.hms.harvard.edu/) that the abundance of
tryptophan in the E. coli proteins is around 1.1%. The data above imply that there are of the
order of 14.256 million tryptophan molecules assembles in the E. coli proteins at any given
time. If we further consider that all the proteins in a bacterium have to be doubled before it
duplicates (every 40 min), then the average tryptophan consumption rate is
ρmax 360, 000 molecules/min.
Since this consumption rate cannot be maintained when the tryptophan level is too low, we
assumed that the consumption rate for this amino acid is given by
ρ( T ) = ρmax
with
T
,
T + Kρ
Kρ = 1, 000 molecules.
This choice for Kρ guarantees that the tryptophan consumption rate is most of the time very
close to ρmax , except when there are of the order of a few thousand molecules of the amino
acid.
According to the website E. coli Statistics (http://redpoll.pharmacy.
ualberta.ca/CCDB/cgi-bin/STAT_NEW.cgi), the average tryptophan molecule count
in this bacterium is
T ∗ 80, 000 molecules.
From Morse et al. (1968), the average number of anthranilate synthase enzymes in E. coli is
E∗ 2, 000 molecules.
Caligiuri & Bauerle (1991) found from their experimental data that the probability that an
anthranilate synthase enzyme is not feedback inhibited by tryptophan can be approximated
by the following function:
Kn
PI ( T ) = n I n ,
KI + T
with
K I 2, 500 molecules,
and
n 1.2.
Let k T be the tryptophan synthesis rate per anthranilate synthase molecule. Given, that the
average tryptophan synthesis rate must equal the consumption rate for this amino acid, we
can solve for k T from the following equation k T E∗ PI ( T ∗) = ρmax . After doing the math we
obtain
k T 12, 000 molecules/min.
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Consider the promoter model discussed in Section 3. Let Pact be the probability that the
promoter is in the global permissive state. We have thus that, in the stationary state,
k+ R A Pact = k− (1 − Pact ). Then, by solving for Pact in the previous equation and taking into
consideration that, when the promoter is in the global permissive state, the probability that
it is ready to be bound by a polymerase is PR . The stationary promoter activity level as a
function of the tryptophan molecule count is
1
P ( T ).
1 + k+ ( T ) R A ( T )/k− R
It is straightforward to test that the promoter activity level equals one when T = 0 molecules.
On the other hand, Yanofsky & Horn (1994) measured that the operon expression level
is maximal under conditions of tryptophan starvation, and that the repression regulatory
mechanism decreases the promoter activity level by 60 times when the tryptophan level
reaches its normal value. Thus, we must have that
1
P ( T ∗ ) = 1/60.
1 + k+ ( T ∗ ) R A ( T ∗ )/k− R
This last result can then be used to estimate parameter KT —see Eqn. (4). Thus, from Eqn. (4)
and given that (Gunsalus et al., 1986)
R T 400 molecules,
we obtain after some algebra that
KT 1.7 × 106 molecules.
Grillo et al. (1999) estimated the following values for the promoter-repressor interaction rates:
−1
−1
k+
i 8.1 molecules min ,
−1
k−
i 6.0 min ,
−1
−1
k+
j 0.312 molecules min ,
−1
k−
j 0.198 min ,
−1
−1
k+
k 0.3 molecules min ,
−1
k−
k 36.0 min ,
k c 40.0
The probability that transcription is not prematurely terminated due to transcriptional
attenuation is given by Eqn. (5). On the other hand, Yanofsky & Horn (1994) measured that
one of every ten polymerases that have initiated transcription finish transcribing the operon
genes when the tryptophan level is at its normal value. This means that PA ( T ) = 0.1. We
obtain from this that
α 2.3 × 10−4 molecules−1 .
Finally, the value of parameter k E is chosen so that, when T = T ∗ , the average enzyme
molecule count is E∗ . We found by inspection that
k E 30, 000 molecules/min.
complies with this requirement.
Transcriptional Bursting
in
the Tryptophan
OperonOperon
of E.
coli
Its Effect
on the
System
Transcriptional
Bursting in the Tryptophan
of E.
Coli and
and its Effect
on the System
Stochastic
DynamicsStochastic Dynamics
193
15
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7
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by
Nitroimidazole and Human Liver Regeneration
Rakesh Sharma1,2
1Center
of Nanomagnetics and Biotechnology,
Florida State University, Tallahassee, Florida
2Amity Institute of Nanotechnology, Amity University, NOIDA UP,
1USA
2India
1. Introduction
Enzymes so called (Enz’-a-ai-am) are biologically active protein molecules responsible of
biochemical reactions in the bacteria, cells and organs. Enzymes regulate the rates of
biochemical reactions to maintain the metabolism to keep active physiological actions in the
body. High enzyme activities or high rates of reactions cause higher product formation or
deposits to initiate disease. Drugs are used as enzyme competitors to normalize the
reactions to correct disease. Most of diseases are cured by ‘enzyme inhibition’. Enzyme
inhibition can be three types: competitive, non-competitive, and uncompetitive. Enzyme
inhibition also provides a kind of defense in cells by regulation of metabolism to inhibit or
stimulate the biochemical processes. Most of the drugs undergo detoxification and
biotransformation in liver to regenerate or formation of new hepatocytes and Kupffer cells.
Major biochemical events in liver regeneration are regulated by enzymes in energy
metabolism, growth factors and cytokine molecules.
Present chapter describes an example of liver cell enzyme battery to regulate the carbohydrate,
lipid and protein metabolism in liver cells, mainly hepatocytes and Kupffer cells in the light of
liver damage due to amoebic liver abscess and role of enzyme inhibition in liver regeneration
by 2’-nitroimidazole. Liver damage by amoeba is manifested by elevated enzymes in cells. As
a result, two major clinical manifestations of Entamoeba histolytica infection are amoebic colitis
and amoebic liver abscesses. To cure amoebic liver abscess, liver regeneration and amoebic
killing by 2’-nitroimidazole therapy is routine in clinical practice. 2’-nitroimidazole acts in liver
to perform enzyme inhibition at the level of carbohydrate, lipid, and protein metabolic
regulatory steps. Earlier, nitroimidazole derivatives were considered drug of choice in
treatment of hepatic hypoxia (low oxygen) conditions in parasitic infections, cancer and
recently nitroimidazole derivatives are emerging as hypoxia markers and radiosensitizers in
tumor treatment [Sharma 2001, Sharma 2011a, 2011b].
Since 1960, hepatocytes were investigated rich in major enzymes regulating the energy
metabolism located in cytoplasm and mitochondria for glycolysis, TCA cycle while drug
196
Enzyme Inhibition and Bioapplications
metabolizing and redox enzymes are located in lysosomes and microsomes. Other enzymes
also participate in defense processes such as respiratory burst, HMP shunt, oxygenase
pathways and inflammatory cytokines. Present chapter reviews the ongoing developments
with clear and complete information on action of new nitroimidazole derivatives as ‘enzyme
inhibitors’ in liver selective cytotoxicity, oxygen depletion, hepatocellular DNA and enzyme
normalization before enzyme can be used in hypoxia monitoring and therapy [Sharma,
2011b]. The paucity of information on nitroimidazole-liver tissue interaction is poor and
available data of initial sequential biochemical changes in liver cells is scanty which further
leads to detectable hypoxia [Sharma 2011a, 2011b]. It is believed that initially enzyme
regulated glucose and calcium hemostasis are the primary targets of hepatic hypoxia
followed by enzyme regulated induced metabolic integrity loss leads to apoptosis,
regulatory failure in glycolytic, TCA cycle, gluconeogenesis and Ca++ mediated cAMP
related biodegradation of molecules [Sharma et al.2011c].
The present chapter focuses on hepatic enzyme inhibition by nitroimidazole at different
levels of energy metabolism, respiratory burst and drug metabolizing enzymes. In following
section, a molecular basis of enzyme inhibition and hepatocellular hypoxia and dysfunction
criteria is described in detail. In following section, regulatory behavior of rate limiting
enzymes of glycolysis, TCA cycle, phagocytosis is described with examples: glucokinase,
phosphofructokinase, pyruvate kinase, lactate dehydrogenase, NADPH cytochrome P450
reductase, phosphodieaterase, lysosomal enzymes in serum, liver biopsy samples. In next
section, role of enzymes in liver abscess is described.
1.1 Enzymes in liver abscess
Entamoeba histolytica is a human-specific pathogen and reproducible animal models of
intestinal amoebiasis have proved elusive to describe interaction between E. histolytica and
liver cells. The most common extraintestinal manifestation of disease is amoebic liverabscess. Amoebic liver-abscesses arise from haematogenous spread (probably via the portal
circulation) of amoebic trophozoites that have breached the colonic mucosa.[Thompson et
al. 1985, Abuabara et al. 1982, Shandera et al. 1998, Barnes et al. 1987, Adams 1977, Lancet
2003]. Effective nitroimidazole treatment and rapid diagnosis showed the mortality rates to
1–3%. Entamoeba histolytica trophozoites can lyse neutrophils in vitro, causing them to release
toxic substance killer enzymes such as superoxide dismutases, collagenases, elastases and
cathepsins [Jarumilinta et al. 1964, Guerrant et al. 1981]. On these lines, author proposed a
‘Hepatocellular dysfunction criteria’ to make systematic observations in abscess
development and step by step method of medical/surgical intervention [Sharma et al.2011].
In previous reports, patient symptoms, leucocytosis without eosinophilia, mild anaemia,
raised concentrations of alkaline phosphatase, and a high rate of erythrocyte sedimentation
were the most common laboratory findings.[ Thompson et al. 1985, Abuabara et al. 1982,
Shandera et al. 1998, Barnes et al. 1987, Adams 1977, Lancet 2003].
Amoeba cause amoebic liver abscesses, which are circumscribed regions of dead
hepatocytes, liquefied cells and cellular debris surrounded by a rim of connective tissue,
some inflammatory cells and few amoebic trophozoites [Sharma et al. 2011]. The adjacent
liver parenchyma is usually completely normal. At same sites in liver so called ‘acini’ fixed
sinusoidal phagocytic cells (Kupffer cells) provide defense against any bacteria, virus or
foreign drug. Kupffer cells have two intracellular structures filled with enzymes: lysosomes
(Lai-zo-somes) and microsomes (Mai-kro-zomes). Lysososmal bags filled with a large
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
197
number of lysosomal enzymes acting as destroyers actually lyse and digest the bacteria,
virus and toxicants. Microsomal enzymes are active in detoxification of drugs or drug
biotransformaton to interact (stimulation or inhibition of enzymes) with metabolic
regulation in liver cells. Let us introduce a bit of enzymes in liver cells performing different
metabolic regulatory functions in following description. Abscesses occupying large areas of
the liver can be cured without drainage, and even by one single dose of
nitroimidazole.[Powell et al. 1969, Lasserre et al. 1983, Akgun et al. 1999]. Today, role of
ultrasound or percutaneous therapeutic aspiration guided by CT in the treatment of
uncomplicated amoebic liver-abscess by surgical drainage is controversial.
Enzyme
Glucose 6 Pase
Glucose 6 PDH
Phoshogluconate DH
Phosphofructokinase
Aldolase
Pyruvate kinase
Pyruvate DH
LDH
Citrate Synthase
Isocitrate DH
Succinate DH
Malate DH
Cytochrome C Oxidase
NADPH Cyt C Reducatase
NADH Oxidase
Tyrosine Aminotransferase
Aniline Hydroxylase
Aminopyrine demethylase
Acid DNAase
Glutathione Reductase
Peroxidase
Catalase
Superoxide dismutase
Guanase
Adinosine Deaminase
Leucine aminopeptidase
Ca++ ATPase
5’ Nucleotidase
Acid Lipase
Acid Phosphatase
Alkaline Phosphatase
Beta glucuronidase
Amoeba
++++
++++
+++
+++
++
++
++
+
++++
+++
++
+++
+
+
+
++
++
++
++
++
+++
++
++
++
+
++
++
++
++
++
+++
++
Hepatocyte
++
++
+
++
++
+
++
++
+
++
+
++
++
+++
++
+
+
+
+
+
+
+
+
++
+
+
+
+
+
+
+
++
Kupffer cell
+
+
+
+
+
+
+
+++
++
++
+
++
++
++
++
++++
++++
++++
++++
++++
++++
++++
++++
+++
+++
++++
++++
++++
++++
+++++
+++++
+++
Source: Sharma R. Effect of Nitroimidazole on isolated liver cells in development of amoebic liver
abscess. Ph.D dissertation submitted to Indian Institute of Technology, Delhi, 1995.
Table 1. Carbohydrate metabolizing enzymes in amoeba, hepatocytes and Kupffer cells.
Abundance of enzyme activities in cells are shown. Comparative enzyme database of
amoeba and isolated liver cells is shown and sketched in Figures 3,5, and 6. Strengths of
different enzyme activities in amoeba, liver cells are shown as weak(+), moderate(++),
high(+++), extreme(++++ or more).
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Enzyme Inhibition and Bioapplications
Characterization of enzymes in amoeba has been a long time quest to explore the possibility of
amebic enzyme inhibition by new drugs in drug discovery. Several antiamoebic drugs are in
market as potent enzyme inhibitors since last four decades. Author proposed a comparative
enzyme database of amoeba and hepatocellular cells in culture (see Table 1 and Figure 1). Of
specific mention, cysteine proteinase enzymes are secreted in large quantities by the parasite
during immune response and can cleave extracellular matrix proteins, which might facilitate
amoebic invasion[Scholze et al. 1988, Keene 1986]. Mainly carbohydrate metabolizing, energy
metabolizing enzymes and lyssomal enzymes are abundant and participate in amoebic –host
liver cell interaction as shown in Figures 2,3a,3b and 3c. These proteins act as virulence factors
in animal models of amoebic liver abscess [Stanley et al.1995, Ankri et al.1999]. A family of six
genes encodes E. histolytica cysteine proteinases (ehcp1–6 ) but ~90% of the proteinase activity is
related to three proteinase enzymes, EhCP1, EhCP2 and EhCP5 [Bruchhaus, et al. 2003].
Although targeted disruption of selected E. histolytica genes has not yet been achieved, stable
episomal expression of foreign DNA is possible in amoebae by maintaining continuous
selective pressure [Hamann et al. 1995, Vines et al. 1995]. This has enabled the investigators to
target specific molecules in E. histolytica by the episomal expression of antisense mRNA or of
genes encoding dominant negative mutants [Ankri et al. 1998]. The antisense approach has
been applied to E. histolytica cysteine proteinases and episomal expression of an antisense
RNA to ehcp5 could reduce total amoebic proteinase activity by 80–90% [Ankri et al. 1998]. To
assess the role of E. histolytica cysteine proteinases in amoebiasis, in earlier study, human
xenografts in SCID-HU-INT mice were infected with amoebic trophozoites expressing the
ehcp5 antisense RNA(proteinase-deficient amoebae) or amoebic trophozoites containing the
same plasmid without the antisense insert (the control group)[Zhang et al. 2000]. Major
findings were: 1. no obvious defect was apparent in the ability of cysteine-proteinase deficient
amoebae to inhabit and survive within the colonic lumen; 2. post-24 h after infection, cysteine
proteinase-deficient amoeba had, in contrast to the control group, failed to induce significant
amounts of human IL-1αor IL-8 from infected intestine; 3. gut inflammation was also reduced
in human intestine infected with cysteine-proteinase-deficient E. histolytica trophozoites; 4.
control E. histolytica trophozoites damaged the intestinal permeability barrier at 24 h but there
was only a minimal increase in intestinal permeability in human intestinal xenografts infected
with cysteine-protease-deficient amoeba; 5. histological studies at 24 h, human xenografts
infected with control amoebae showed damage to the colonic mucosa, invasion of amoebic
trophozoites into submucosal tissues and neutrophil-predominant inflammation; 6. by
contrast, xenografts infected with cysteine-proteinase-deficient amoebae showed less mucosal
damage, almost no evidence for amoebic invasion into submucosal tissue and little
inflammation. Authors speculated that cysteine-proteinase-deficient amoeba might have a
defect in their ability to induce gut inflammation and to invade into the submucosal tissues
[Zhang et al. 2000]. How amoebic cysteine proteinases contribute to gut inflammation and
tissue damage in amoebiasis? Possibly, Entamoeba histolytica trophozoites expressing the ehcp5
antisense RNA might have reduced the phagocytic capabilities or reduced virulence in
amoebiasis [Ankri et al.1998]. However, protease-deficient amoebae maintain their ability to
lyse target cells, so a defect in cell killing does not underlie the reduced virulence of cysteine
proteinase-deficient amoebae [Seydel et al.1997]. In addition, it is unlikely to be a direct effect
of cysteine proteases on intestinal epithelial cells, because amoebic lysates rich in cysteine
protease activity fail to induce high levels of cytokine production or inflammation when
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
199
placed in human intestinal xenografts [Ankri et al.1998]. It suggests that amoebic cysteine
proteinases must exert their effects on gut inflammation and tissue damage after an initial step
that requires live active trophozoites.
1.2 Entamoeba histolytica and programmed cell death
Other important role of caspase enzymes was explored when E. histolytica trophozoites were
incubated with common types of mammalian cells. Normally trophozoites kill mammalian
cells in a contact dependent manner by two processes: by lytic necrosis [Berninghausen et
al.1997] or undergo apoptosis[Ragland et al.1994]. Apoptosis, or programmed cell death, is
an ordered system of cell death with an initiator or signaling phase followed by an effector
stage that causes cell death by degrading various cellular components. The effector stage
involves the activation of caspase enzymes, cysteine proteinases with specificity for
aspartate residues that form a cascade, converging on caspase 3. A key biochemical marker
of apoptosis is endonuclease cleavage of chromatin between histone bodies, which can be
seen as a DNA ladder on separating gels.
Detection of DNA fragmentation in situ by the endlabeling of single- or double-stranded
DNA breaks with terminal deoxyribonucleotidyltransferase (TUNEL assay) has also been
used as a biomarker of apoptosis [Schulte-Hermann et al.1994]. However, E. histolytica can
induce apoptosis in vitro raises doubt if apoptosis is a component of the cell death
commonly seen in amoebic liver abscess. To test this fact, amoebae were inoculated into the
livers of SCID mice and evidence for apoptosis was sought by gel separation of DNA from
infected livers and TUNEL staining of amoebic liver abscesses [Seydel et al.1998]. DNA
ladder formation was detected in samples obtained from SCID mice by 1 h after amoebic
inoculation. TUNEL staining revealed significant areas of apoptosis within amoebic liver
abscesses common in inflammatory cells and hepatocytes close to amoebic trophozoites
[Stenley, 2001]. However, some distant liver cells also were TUNEL positive. Thus, in the
murine model of amoebic liver abscess, E. histolytica induced cellular apoptosis appears to
be a significant component of cell death as shown in Fig. 3. Interestingly, E. histolyticainduced apoptosis was not blocked by Bcl-2 [Ragland et al.1994] and did not appear to
involve either of the two of the major pathways for the induction of apoptosis: ligation of
the Fas receptor and ligation of TNFα1 receptor 1 (TNFR1). In both C57Bl/6 MRL-lpr/lpr
mice with no Fas receptor, and C57Bl/6.C3H-gld/gld mice with no Fas ligand, apoptosis
was detected in amoebic liver abscesses by DNA ladder formation and TUNEL staining
[Seydel et al. 1998]. Apoptosis was also present in hepatocytes in amoebic liver abscesses
from both TNFR1 knockout mice and the heterozygote controls[Seydel et al. 1998].]. These
data indicate that E. histolytica causes apoptosis in mouse liver by a mechanism that is
independent of Fas–Fas-ligand interactions and TNFR1.
Genetic approaches and new models of amoebic liver abscesses have provided good
prospects on the complex interactions between E. histolytica and the host liver cells [Hamann
et al.1995]. Well-known E. histolytica enzymes lyse cells and digest extracellular matrix
proteins. Enzymes have capability to induce apoptosis in hepatocytes and inflammatory
cells. Enzymes can stimulate, and perhaps enhance, an NF-κB-mediated human liver
inflammatory response that contributes to tissue damage. These new enzyme regulated
pathways might offer an explanation for some of the clinical and pathological differences
200
Enzyme Inhibition and Bioapplications
between amoebiasis and amoebic liver abscess. The hepatocyte inhabited apoptosis would
predominate in amoebic liver abscesses. If these mechanisms prove to be important in
human disease, they will provide new targets in both the host and the parasite for
interventions designed to ameliorate or inhibit amoebiasis and amoebic liver abscesses.
Recently, author in team established that apoptosis results with hypoxia in liver cells and
evaluated nitroimidazole cytotoxicity [Kwon et al. 2009]. In following section, a criterion of
hepatocellular hypoxia is described based on liver cell enzymes and cytomorphology.
Fig. 1. Entamoeba histolytica trophozoites induce apoptosis in inflammatory cells and
hepatocytes in amoebic liver abscesses. Section of liver stained for apoptotic cells with the
TUNEL method from an amoebic liver abscess in a C57Bl/6 MRL-lpr/lpr mouse. Amoebic
trophozoites (long arrow) are adjacent to a cluster of inflammatory cells (short arrow),
which show marked TUNEL staining. TUNEL staining in nuclei and cytoplasm is also
evident in a band of dead hepatocytes and inflammatory cells (D). TUNEL-positive nuclei
(brown staining) are also visible in hepatocytes in regions (H) flanking the dead cells.
Reproduced, with permission, from Ref. Seydel et al. 1998.
2. Liver enzymes and enzyme inhibition in human liver: Hepatocellular
criteria
The liver is made of parenchymal hepatocytes and nonparenchymal Kupffer cells as sole
targets that exhibit their intracellular biochemical changes as hepatocellular enzyme
biomarker profile. Liver cells are rich in enzymes. Enzyme inhibition in liver is characterized
by two ways: 1. intact hepatocellular enzymes in hypoxia serum and liver homogenates; 2.
Enzyme regulatory behavior by characterizing enzymes in presence of additives added to
cultures of isolated liver cells. Additives and drugs are mainly biotransformed and
detoxified in liver by a battery of drug metabolizing enzymes: lysosomal and microsomal
enzymes. Drugs inhibit enzymes. In following description, readers are introduced with the
importance of enzyme inhibition as research tool for two purposes: 1. drug testing and
hypoxia disease monitoring (Hepatocellular Hypoxia Criteria) to understand the hypoxia
and liver cell interactions with amoeba and nitroimidazole; 2. Enzyme regulatory behavior
in presence of additives in isolated liver cells in culture (hepatocytes and Kupffer cells).
2.1 Hepatocellular hypoxia criteria
Author proposed a “Hepatocellular Hypoxia Criteria”. ‘Hepatocellular hypoxia criteria’
assumes that initially liver cells loose metabolic integrity (ATP and NADPH insufficiency
from glucose to cause oxygen insufficiency in mitochondria) and undergoes apoptosis
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
201
followed by detectable necrosis in liver. Hypothesis was that liver cells undergo a series of
changes - hepatocytes undergo metabolic energy loss and oxygen depletion (hypoxia) while
Kupffer cells undergo phagocytosis, respiratory burst and proteolysis. Nitroimidazole
induces enzyme inhibition in liver cells to normalize the elevated enzymes during metabolic
energy loss and oxygen depletion (hypoxia) in liver cell (in mitochondria) and lysosomal
enzyme inhibition to combat the phagocytosis. The approach is applicable possibly in
design of drugs to treat oxygen starved tumor cells or infected liver lesions and abscesses.
Previous reports on first initial stage of nitroimidazole induced liver cell cytotoxicity
indicated the apoptosis, glutathione, DNA interaction, oxygen depletion, lactate inhibition,
enhanced NADPH cytochrome reductase and superoxide dismutase enzymes [Ersoz et
al.2001, Brezden et al.1997, Noss et al.1988, Sugiyama et al.1993, Berube et al. 1992, Sapora et
al.1992, Moselen et al. 1995, Stratford et al.1981, Adams et al.1980, Noss et al. 1989, Mulcahy
et al.1989, Widel et al. 1982, Rauth et al. 1981, Chao et al. 1982, Whitmore et al.1986]. Still
nitroimidazole structure-function relationship between hypoxia, radiosensitization and
cytotoxicity or liver regeneration remain less understood perhaps due to nitroimidazole low
sensitivity to clinical investigations and significant enzyme alterations only at the very late
necrotic stage [Cowan et al.1994, Melo et al. 2000, Moller et al. 2002, Ballinger et al. 1996,
Edwards et al. 1981, Edwards et al.1982, Ballinger et al.2001, Riche et al.2001, Melo et
al.2000]. However, nitroimidazole cytotoxicity was reported continuously in last three
decades indicating many altered enzymes involved in nitric oxide production, cytokines,
oxygen depletion (hypoxia) and immunoactive substance release from both hepatic
infections and cancer tumor tissues [Su et al.1999, Rumsey et al.1994, Koch et al.2003,
Hodgkiss et al.1997, Sharma 2009a, Sharma et al.2009b]. In following section, author puts an
evidence of hepatic metabolic integrity loss (in mitochondria and cytoplasm) and lysosomal
stimulation associated with hypoxia in liver. Nitroimidazole is still considered clearly
hepatotoxic, renal and neurotoxic drug with several reports of diffused damage of
parenchymal and nonparenchymal liver cells as initial stage of hepatic damage observed by
electron microscopy and biochemical markers in serum with emphasis of pathophysiology
[Mahy et al.2004]. Our previous observation on nitroimidazole induced effects to act against
hepatic amoebiasis and amoebic abscess development indicated the possibility of
nitroimidazole concentration-dependent effects upon oxygen depletion, nitric oxide
production, cytokine synergy and liver cell enzyme alterations as inter-related consequences
of liver regeneration or bringing back elevated enzyme levels [Chu et al.2004]. The major
players were the energy metabolizing and drug metabolizing lysosomal enzymes in initial
liver damage with assumption that liver cell enzyme profile defines the hypoxia sensitivity
of nitroimidazole by ‘Hepatocellular Hypoxia Criteria’. The novelty of hepatocellular
hypoxia criteria was detail cytomorphic-biochemical information extracted from altered
enzyme activities during initial cytomorphic changes in hypoxic hepatic cells
simultaneously with microstructure changes by electron microscopy to confirm the role of
cell organelle in hypoxia development. The criterion can be used to evaluate common liver
hypoxia conditions such as hepatic tumors, hepatic infections etc. Several new
nitroimidazole derivatives are emerging as potential cancer chemosensitizers, hypoxia
markers and hypoxia imaging contrast agents acting as enzyme inhibitors [Gronroos et
al.2004, Ziemer et al.2003, Jankovic et al. 2006, Papadopoulou et al.2003, Eschmann et
al.2005, Heimbrook et al.1988, Berube et al.1992, Sharma et al. 2009].
202
Enzyme Inhibition and Bioapplications
Components of Hepatocellular Hypoxia Criteria: The criterion is described a step by step
sequence of oxygen depletion in hepatocellular damage: 1. initial loss of metabolic integrity;
2. programmed regulatory failure of cell oxygen and energy metabolism; 3. liver tissue
inflammation and immunity loss; 4. necrosis and active death. The purpose of metabolic
integrity in hepatocytes is keeping cells intact by maintaining balance between glucose
formation and glucose breakdown to maintain energy flow of NADPH and ATP molecules.
Sequence: The nitroimidazole induced liver cell cytotocity leads to enhanced glucose
breakdown and more demand of ATP and NADPH. With course of time, high energy
demand at the cost of cell metabolic resources leads to metabolic integrity loss or energy loss
and oxygen deprivation followed by possibility of step by step programmed cell death (only
in tumor cells). In normal liver cells, the nitroimidazole cytotoxicity effect cause less damage
and cells sustain the effect while struggling oxygen starved infected or tumor cells further
loose capability to stay alive (such cells die after nitroimidazole induced infected or tumor
cell killing). In normal cells, liver regeneration recovers the damage.
Hepatic hypoxia criterion has 3 major enzyme components: 1. major event of high glycolytic
rate is immediate result of high glucose turnover such as glycolysis followed by secondary
metabolic cycles viz. tricarboxylic acid, glycogenolysis, gluconeogenesis, pentose phosphate
pathways; 2. changes in peripheral biomarkers such as cytokine immunity, altered
glutathione reductase, nitric oxide production, super oxide dismutase enzymes; 3. after
severe energy loss, changes occur in liver cellular morphology and tissue shrinkage [Sharma
et al.2011, Kwon et al. 2009]. The sensitivity of different enzymes as cytomorphic
manifestation of hypoxia in liver abscess and nitroimidazole induced liver regeneration is a
simple scheme of “Hepatocellular Hypoxia Criteria” in steps is shown in Table 2. In
following section, different enzymes are described.
Morphological changes
Clinical changes
Liver biochemical changes
1. Physical examination:
--
abdominal pain
--
intestinal damage
fever
--
hepatic infiltration
hepatomegaly
liver function tests(elevated)
Loss of cellular metabolic integrity
2.Electron microscopy:
hepatomegaly with
Mitochondria(M)
diffused injury
endoplasmic reticulum(ER)
altered hepatocyte enzymes of:
low ATP/ADP; NADPH/NADP
gluconeogenesis
peroxisome (P)
glycogenolytic
lysosome (L)
lysosomal enzymes
nuclear changes (N)
oxygen flux related
Hepatocellular enlargement (Apoptosis)
Table 2. Continued
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
Morphological changes
Clinical changes
3. Cellular organelle damage:
poor drug response
Liver biochemical changes
slow metabolic disorder:
mitochondria(M)
oxidative phosphorylation
microsomes (MI)
drug metabolizing enzymes
lysosome (L)
initial phagocytosis
nuclear (N)
DNA fragmentation(beads)
cytosol (C)
203
glucose/protein/respiratory burst
Hepatocellular Oxygen insufficiency (inflammation)
4. Liver pathology changes:
raised diaphragm
mitochondrial damage
amebic liver scan +ve
stimulation of Kupffer cells
exfoliative ER
hyperplasia of Kupffer cells
anisonucleosis
loss of metabolic control
autophagy & lysosomal
irritation
increased water accumulation
cytosolic granulation
increased molecule imbalance
fatty liver appearance with
increased lipid synthesis
membrane damage
Hepatocellular degeneration and necrosis
5. Hepatocytology:
Cell proliferation
advancing tumor
vascularization
surgical aspirates (altered
Cell debris
tissue growth on
ultrasound
proteins, lipids, enzymes)
Nitroimidazole single dose therapy schedule
6. Liver cell recovery
negative liver
scan/ultrasound
normal liver function test
Tissue shrinkage
Hypoxia monitoring
positive
-ve Hypoxia biomarkers
if unchanged or poor
recovery
abnormal ELISA,enzymes
OR
Surgical intervention
Source: Sharma R. Effect of Nitroimidazole on isolated liver cells in development of amoebic liver
abscess. Ph.D dissertation submitted to Indian Institute of Technology, Delhi, 1995.
Table 2. A step by step scheme of “hepatocellular hypoxia criteria” to evaluate liver hypoxia
damage in infected hepatitis or hepatic tumors. Different clinical methods suggest
composite picture of hepatic hypoxia and associated biochemical and cytomorphic changes.
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Enzyme Inhibition and Bioapplications
2.2 Origin of hypoxia and enzyme regulation
Oxygen insufficiency or hypoxia begins with low NADPH and ATP supply from glucose.
Glucokinase, aldolase, pyruvate kinase and lactate dehydrogenase possibly serve as initial
glycolytic regulatory enzymes for energy flow to generate tricarboxylic acid cycle (TCA)
precursors. TCA cycle being as main source of energy molecule synthesis and
gluconeogenesis, it serves as sole source of central ATP pool in glucose homeostasis during
liver cytototoxicity or regenerated liver cell [Sharma et al.2007, Sharma et al.2010]. The
citrate synthase, malate dehydrogenase, isocitrate dehydrogenase and succinate
dehydrogenase enzymes serve as TCA cycle regulatory enzymes. These enzymes in
hepatocytes characterize the tumor hypoxia and hepatocyte response after nitroimidazole
therapy [Horlen et al.2000, Tabak et al.2003, Sapora et al.1992, Moselen et al.1995, Stratford
et al.1981]. At the cost of ATP from TCA cycle and oxygen from the cell, oxidative
phosphorylation maintains the metabolic integrity and continuous flow of energy. In the
state of low NADPH and low oxygen cell undergoes state of metabolic integrity loss and
“hypoxia”. NADPH dependent cytochrome redox enzymes are biomarker of oxidative
phosphorylation status and oxygen insufficiency (hypoxia) in liver. Other glutathione
reductase and superoxide dismutase enzymes are biomarkers of hypoxic state of liver cells
[Larrey et al.2000, Michaopoulos et al.1997]. Any energy imbalance and oxygen insufficiency
(hypoxia) in liver cell are indicated by these enzymes [Michaopoulos et al.1997, RamirezEmiliano et al.2007]. The behavior of these enzymes in isolated liver cells is described in
following section on isolated liver cell enzymes.
Fig. 2. The liver scan (on left panel) is shown for assessing the position of hepatic hypoxia
and associated hepatomegaly and for biopsy collection site as shown with arrow.
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Nitroimidazole
Fig. 3a. The histogram bars show the effect of nitroimidazole on biomarker enzymes and
comparison of different enzymes in hepatocytes in control vs nitroimidazole treated subjects
Fig. 3b. The histogram bars show the effect of nitroimidazole on biomarker enzymes and
comparison of different enzymes in Kupffer cells in control vs nitroimidazole treated
subjects in control vs nitroimidazole treated subjects
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Enzyme Inhibition and Bioapplications
Fig. 3c. The histogram bars show the effect of nitroimidazole on biomarker enzymes and
comparison of different enzymes in serum biomarker enzymes.
3. Enzyme inhibition in initial loss of metabolic integrity, glycolysis and ATP
in hepatocytes
Enzymes in serum and liver cells from positive control amoebic liver abscess patients
exhibited more or less specific characteristic changes in enzymes [Sharma et al.2008]. First
goal was to establish elevated enzyme levels in patients and to prove subsequent enzyme
inhibition after nitroimidazole therapy. Author reported ten control subjects with 2’nitroimidazole therapy follow up for their carbohydrate metabolizing enzymes in serum
and hepatocellular enzymes in liver biopsy tissues. Proven ten hepatic hypoxia control
subjects were studied for hypoxia by enzyme assays [Sharma et al.2008]. The 2’nitroimidazole treated paired ten subjects were studied for hypoxia related enzyme
inhibition using enzyme assays and hepatocellular cytomorphology by electron microscopy.
Out of ten positive control subjects, nine patients showed elevated carbohydrate
metabolizing and lysosomal enzyme levels in serum: Glucokinase (in 80% samples),
aldolase (in 80% samples), phosphofructokinase (in 80% samples), malate dehydrogenase (in
75% samples), isocitrate dehydrogenase (ICDH) (in 60% patients) were elevated while
succinate dehydrogenase and lactate dehydrogenase (LDH) levels remained unaltered.
Lysosomal β-glucuronidase, alkaline phosphatase, acid phosphatase enzymes showed
enhanced levels in the all patient serum samples. In ten control liver biopsies, the isolated
hepatocytes and Kupffer cell preparations showed altered liver cell enzyme levels.
Hepatocytes showed reduced glucokinase (in 80%), LDH (in 80%), and higher content of
Mechanisms of Hepatocellular Dysfunction
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aldolase (in 80%), pyruvate kinase (in 100%), malate dehydrogenase (in 80%), ICDH (in
80%), citrate dehydrogenase (in 70%), phosphogluconate dehydrogenase (in 80%). Kupffer
cells showed higher enzyme levels of β-glucuroronidase (in 80%), leucine aminopeptidase
(in 70%), acid phosphatase (in 80%) and aryl sulphatase (in 88%). In these 10 biopsy samples
from subjects, the electron microscopy cytomorphology observations showed swollen
bizarre mitochondria, proliferative endoplasmic reticulum, and anisonucleosis. In 2’nitronidazole clinical trial patients, after 2’-Nitroimidazole effect in liver cell damage was
manifested as enzyme inhibition in liver cells as shown in Figure 3[Sharma 2008].
Nitroimidazole treated patients shows altered enzymes: glucokinase levels were more or
less normal in both serum and liver biopsy (100%); phosphofructokinase levels were
nonspecific in nitroimidazole treated subjects or normal (80%); elevated lactate
dehydrogenase levels were reversed to normal. In nitroimidazole treated subjects the
elevated enzyme levels due to amoebic abscess brought back to normal after nitroimidazole
treatment due to enzyme inhibitory effect of nitroimidazole: isocitrate dehydrogenase
enzyme normal levels (80 %); the citrate synthase normal levels (80%); phosphogluconate
dehydrogenase normal levels (80 %). In nitroimidazole treated subjects, the succinate
dehydrogenase levels were normal (60 %) in both. The enzymes are shown in Figure 3 and
Tables 2-4.
3.1 Enzyme inhibition in Kupffer cell lysosomal enzymes and Hypoxia
In nitroimidazole treated patients showed elevated enzymes brought back to normal: βglucuronidase normal levels (50 %) in Kupffer cells and normal in serum (60 %); acid
phosphatase normal levels in both; leucine aminopeptidase levels were normal in serum (80
%) and remained high in Kupffer cells (60%); Guanase levels were normal in both Kupffer
cells and serum (80%). The enzymes are shown in Figure 2 and Table 3.
Kupffer cell hyperplasia was observed with swollen lysosomal contents as shown in Figure
4. Altered glycolytic enzymes in cytosol, TCA cycle enzymes in mitochondria, lysosomes
and increased synthesis of enzymes by endoplasmic reticulum showed correlation with
clear liver cell degeneration of microbodies. After nitroimidazole treatment, observations of
both electron microscopy and biochemical parameters suggested the reversed hepatocellular
changes towards normal recovery or liver regeneration. In following section, I describe the
outcome of nitroimidazole effect in liver regeneration and enzyme inhibition as defense.
The enzyme biomarkers could be analyzed in serum and liver cells (hepatocytes and
Kupffer cells) as clinically significant indicator of hepatic damage in disease or cytotoxicity
of drug. We assume that initially metabolic integrity loss leads to over-secretion of liver cell
enzymes including lysosomal enzymes. Soon after, the ultrastructural changes in liver cells
become evident by electron microscopy suggestive of acute organelle degeneration.
Ultrastructural hepatocyte cytotoxicity was associated with nitroimidazole overdosage
(normal dosage is 2 × 3 gm one time in amoebic hepatitis and thrice in amoebic abscess). The
regenerative changes were consistently observed after nitroimidazole therapy to reverse
liver damage. Few pathology reports are available to show nitroimidazole cytotoxicity and
no electron microscopy study is available to support asymptomatic inflammatory or
unequivocal diffuse parenchymal injury exhibiting diffused sinusoidal and portal
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Enzyme Inhibition and Bioapplications
infiltration events [Tabak et al.2003, Larrey et al.2000]. Biomarker liver cell enzymes with
electron microscopy put evidence of amoebic cytotoxicity in liver biopsy samples and liver
regeneration after nitroimidazole treatment by ‘hepatic hypoxia criteria’. Degenerative
changes of hepatocytes suggested possible necrosis.
Fig. 4. Ultrastructural changes are shown in hepatocyte organelles during hypoxia:
exfoliation of endoplasmic reticulum (top on left); anisonucleosis (mid and bottom on left);
intercellular junction gaps (top and mid panels in center); nuclear inclusions (bottom on
center); swollen and bizarre mitochondria(top on right); inclusions in peroxisome (mid on
right); mitochondrial atrophy with lipid vesicles (bottom on right). Hepatocyte
mitochondria became swollen, bizarre with dense matrix and showed destorted cristae,
endoplasmic reticulum dilated vesicles, giant nuclei with diffuse proliferation of
endoplasmic reticulum and clear anisonucleosis features. The ultrastructure of liver cells
showed characteristic organelle changes in nitroimidazole treated liver.
Evidence of hepatic regeneration and nitroimidazole cytotoxicity (hypoxia induced changes)
was characterized previously in terms of cytokine synergy, unusual degree of
anisonucleosis, nitric oxide production and the presence of giant nucleus in hepatocytes
after liver regenerative therapy [Michalopoulous et al.1997]. Endoplasmic reticulum showed
a diffuse and intense proliferative activity in these liver cells with normal appearance of
mitochondria as earlier reported [Ramirez-Emiliano et al.2007, Das et al.1999]. However,
intramitochondrial inclusion bodies were absent while they were very prominent features
Mechanisms of Hepatocellular Dysfunction
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[Das et al.1999]. The cause of the ultrastructural changes after nitroimidazole cytotoxicity
were supported by initial enzyme alterations reflecting loss of metabolic integrity probably
induced by free radical formation from nitroimidazole [Das et al.1999]. Since ultrastructural
changes in liver were completely reversible after nitroimidazole therapy within 7-9 days, it
is quite reasonable that pathogenesis of diffused hepatocyte damage was due to
nitroimidazole breakdown products.
Fig. 5. The sketch of nitroimidazole induced intracellular enzyme inhibition changes
indicating the points of metabolism and metabolic control during hypoxia and subsequent
liver recovery after nitroimidazole treatment. The enzymes distribution in organelle and
enzyme location in different hepatic sites explains the liver damage and enzymatic basis of
hepatocellular hypoxia criteria [reproduced from Sharma R. 1990. Ph.D dissertation].
As indicated in previous section, ‘Hepatocellular hypoxia criteria’ is a sequence of events in
hepatic cell injury at molecular level. The initial loss of hepatocellular metabolic integrity
leads to hepatic injury. Glucose-energy metabolic integrity loss and nitric oxide formation
with Ca++ homeostasis are main initial determinants of hypoxia [Ramirez-Emiliano et
al.2007]. In following section, present chapter extends the enzyme regulatory behavior
during glucose metabolizing pathways viz. glycolysis, TCA cycle and gluconeogenesis to
establish the value of enzyme inhibition in liver regeneration. In isolated liver cells,
metabolic alterations of energy metabolism pathways explain the events in hepatitis and
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Enzyme Inhibition and Bioapplications
hepatic tumors as best correlated respective enzyme dysfunctions and ultrastructural
changes observed in cells in biopsy. The following description is broad explanation of
different enzymes secreted from hepatocytes as a result of nitroimidazole induced
cytotoxicity and oxygen depletion. From biochemical stand point, different regulatory
enzymes are discussed as biochemical events of hypoxia development as shown in Figures
3a,3b,3c and 5.
Fig. 6. The sketch of relative biomarker enzyme changes (thickness of arrow) in liver
hypoxia. The +ve sign denotes the relative increase in enzyme activity and –ve sign denotes
the decrease in enzyme activity in liver cells in different organelles due to ameba induced
hypoxia. The figure sketch also represents a relative enzyme activities in sequence of
metabolic steps as ‘hepatocellular hypoxia criteria’. Notice the liver regenerative or enzyme
inhibitory action of nitroimidazole at different enzyme reactions is shown [Reproduced
from Sharma R. 1995. Ph.D dissertation].
Mechanisms of Hepatocellular Dysfunction
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3.2 Role of enzymes in energy metabolic integrity in hypoxia
Hexokinase, a rate limiting enzyme in cytosol for glucose turnover was estimated due to
high glucose conversion into glucose 6P showed high enzyme secretion from hepatocytes.
Aldolase, rate controlling enzyme which splits fructose-1, 6 Diphosphate into two 3phosphoglyceric acid and dehydroxyacetone phosphate, showed high enzyme secretion
from heptocytes. Phosphofructokinase, rate controlling enzyme which phosphorylates
fructose 6P into fructose 1, 6 DiP, enhanced as its more demand in hepatocellular glucose
turnover which probably induced higher secretion of phosphofructokinase. Pyruvate kinase,
transferring phosphoryl group from phosphoenolpyruvate to ADP with pyruvate
formation, showed high enzyme secretion from hepatocytes. It may be attributed due to
high concentration of glycolytic intermediates as terminal step was blocked or due to 2, 3 DiPhosphoglycerate regulated abnormal oxygen dissociation. Malate dehydrogenase and
isocitrate dehydrogenase enzymes, catalyzing malate oxidation into oxaloacetate and
oxidative decarboxylation of isocitrate into α-ketoglutarate respectively both require NAD+.
Both enzymes were elevated in damaged cells due to reducing equivalent low ratio of
NADH/NAD+ pushing in forward direction. Succinate dehydrogenase, oxidizing succinate
to fumerate using FAD, showed decreased enzyme levels due to low iron sulphur proteins
resulting with lowered electron transport system in inner mitochondrial membrane i.e.
supply of electrons to molecular oxygen by electron transfer from FADH2 to Fe+++ (SDH) in
amoebic recruited liver cells. Citrate synthase, synthesizing citrate from oxaloacetate and
acetyl CoA by aldol condensation followed by hydrolysis, showed elevated levels due to
rapid turnover of oxaloacetate and acetyl CoA molecules during cytolysis. Moreover, high
TCA cycle activity in hepatocyte during liver damage conditions was described earlier
[Sharma 2010]. In serum, the enzymes exhibit their significance but aldolase, pyruvate
kinase and LDH, MDH, ICDH observed as distinguishing the diffused injury or abscess
formation [Virk et al.1989]. Phosphogluconate dehydrogenase elevated levels may be
attributed due to ribulose 5P formation and transaldolase and transketolase control, for
phosphogluconate pathway. Nitroimidazole induced cytotoxicity perhaps have insignificant
impact on ATP supply. In following section, different enzyme regulatory behaviors of
nitroimidazole induced enzyme inhibition are described during liver cell regeneration. A
strategy was developed to isolate hepatocytes and Kupffer cells in monoaxenic cultures
from excised liver samples. Mixing different inhibitors or additives in liver cell cultures
stimulate the enzyme regulatory behavior in presence of specific growth factors, metabolite
analogues.
3.3 Role of lysosomal enzymes in hepatocellular damage
In previous section, role of secretory lysosomal enzymes was described in defense of liver
cells in context of intestinal amoebiasis and amoebic liver abscess. Conceptually, lysosomal
hydrolases cause foreign body damage by several reactions including redox reaction,
phosphorylation
reaction,
glucuronidation
reaction,
hydroxylation
reaction,
dehydroxylation reaction, nucleation reaction, terminal breakdown reaction, transcarbamylation reaction. Other studies also reported molecular mechanisms of enzymes in
pathogenesis [Das et al.1999], liver cell damage [Virk et al.1989, Virk et al.1988],
phagocytosis [Sharma 2009], cyclooxygenase-2 expression [Sanchez-Ramirez et al.2000].
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Enzyme Inhibition and Bioapplications
4. Enzyme inhibition properties and enzyme regulatory behavior in isolated
liver cells
Isolated liver cells with characteristic enzyme properties are research tools. In our
previous study, the hepatocyte yield from livers was 2.5× 108 per mg liver. The cytosol
fraction showed the removal of all cell organelle from supernatant and resultant
supernatant as cytosol was free of any remnant by microscopic observation by trypan blue
exclusion. Isolation of human Kupffer cells was developed by a modified pronasecollagenase enzymatic isolation and purity of Kupffer cells by using biomarker enzyme
assay. Earlier reports used similar practice of enzymatic digestion and enzymatic assays
as indicators of cell viability and yield [Hendriks et al.1990, Neaud et al.1995, McCuskey
et al.1990]. A new rapid method was used for the isolation and fractionation of both rat
and human Kupffer cells without the need of liver perfusion techniques. The study used
rat livers or small human liver wedge biopsies obtained peroperatively and incubated
with pronase under continuous pH registration. Kupffer cells were subsequently
separated from other nonparenchymal cells by Nycodenz gradient centrifugation and
purified by counterflow centrifugal elutriation [Neaud et al.1995]. Identification of
Kupffer cells was achieved on the basis of ultrastructural analyses and
immunophenotyping [Neaud et al.1995]. The fractionation of Kupffer cells gave good
yield comparable with other studies [Hendriks et al.1990, Neaud et al.1995, McCuskey et
al.1990]. For details of protocol of liver cell isolation, see the Appendix in the end of this
chapter. Human liver cell enzymes in isolated liver cells (hepatocytes and Kupffer cells in
cultures) showed characteristic regulatory behavior in presence of additives.
Liver cell hypoxia is represented as a state of oxygen depletion in cell. Initially liver cell
gets its ATP and NADPH supply from glycolysis and TCA cycle. Consequently, electron
transfer chain (oxidative phosphorylation) through series of cytochrome redox reactions
converts available cell oxygen to water to produce high energy metabolites in the cell
(metabolic integrity). Infected liver cells show loss of metabolic integrity and less
available oxygen (oxygen starved or hypoxia). Nitroimidazole was established two
decades ago as potential oxygen quenching drug in most of the infected and tumor cells.
“Nitroimidazole oxygen quenching action” makes oxygen depleted or starved cells that
further undergo worse to die and leaving normal cells fully functional that can be
observed as potential tumor hypoxia therapy and antiparasitic treatment by
nitroimidazole.
In following section, enzyme inhibition characteristics in energy metabolism regulation
including glycolysis, TCA cycle regulatory enzyme behavior and drug metabolizing
proteolytic lysosomal enzymes induced by additives in liver cell cultures are described.
4.1 Glucokinase
The actinomycin D showed non-significant inhibitory change in glucokinase enzyme
activity as shown in Figure 7 at the top histogram bars. The nitroimidazole showed
enhanced glucokinase activity and reversed the inhibitory effect of actinomycin D. The
glucokinase activity in hepatic abscess hepatocytes did not show any significant effect of
nitroimidazole. Overall actinomycin D alone did not show any change in glucokinase
enzyme activity in hepatic abscess hepatocytes or nitroimidazole treated hepatocytes.
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The addition of triamcinolone with insulin in combination showed significant increase in
glucokinase activity in all hepatocyte groups. It pointed out to the enhancement in hormone
dependent glucokinase enzyme synthesis in hepatocytes and enzyme activity showed highest
enzyme synthesis rate in hepatic abscess recruited hepatocytes while nitroimidazole treated
hepatocytes showed lesser increase in hormone dependent glucokinase in comparison with
hepatocytes as shown in Figure 7. The increase in enzyme activity or enhanced glucokinase
enzyme synthesis in hepatocytes showed that glucokinase enzyme behavior was hormone
dependent. Different concentration of insulin and progesterone hormones further indicated
the regulatory behavior of glucokinase enzyme as hormone dependent.
The addition of progesterone hormone 0.1 µgm/ml in hepatocyte cultures showed
maximum enhanced glucokinase enzyme activity and nitroimidazole treated hepatocytes
showed less enhanced glucokinase activity but more than control hepatocyte enzyme
activity as shown in histogram bars (see Figure 7).
In control group, glucokinase enzyme activity in untreated hepatocytes did not show any
chnage in their glucokinase enzyme activity in presence of nitroimidazole additive. The
yield of hepatocytes and glucokinase enzyme activities from hepatic abscess liver biopsy
were similar as yield of cells and glucokinase enzyme activities in untreated and
nitroimidazole treated hepatocytes. So, it was clear that addition of actinomycin-D and
insulin with triamicilone both enhanced the glucokinase enzyme synthesis in hepatocytes as
shown in Figure 7 histograms bars.
added
added
progesterone
Fig. 7. The nitroimidazole effect on glucokinase enzyme activity in guinea pig normal and
infected hepatocytes is shown in presence of different enzyme regulatory additives
actinomycin D, insulin-triamcilone. Notice the enhanced effect of insulin-triamicilone on
glucokinase enzyme (P value <0.0001; r2 0.9872) in nitroimidazole treated vs liver abscess
biopsy heaptocytes.
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Enzyme Inhibition and Bioapplications
4.2 Pyruvate kinase
The altered behavior of pyruvate kinase regulation in isolated hepatocytes from
nitroimidazole and progesterone induced cultures showed characteristic sigmoid behavior
in normal group but towards linear relationship at increased pH of hepatocyte culture
media. In other words, a pH increase alters the enzyme velocity in same proportion with
ATP concentration changes in linear manner in liver cell medium. At high ATP
concentration, enzyme synthesis in nitroimidazole stimulated cells remained significantly
elevated while at pH 7.0-7.4 cells did not show sudden enzyme elevation and showed a fall
in enzyme activity as observed in normal hepatocytes (see Figure 8). The higher ADP/ATP
ratio kept the enzyme synthesis high at pH 7.4 in progesterone while nitroimidazole
stimulated the hepatocyte cells in cultures (see Figure 8).
The intricacy was reported as ATP interacts with H+ and Mg++ to give complex distribution
of many ionized and metal complex species. However, Mg++ free ADP itself has three
subspecies viz. ADP--, ADPK--, ADPH— whose relative proportions vary with change in pH.
Out of which ADP— happens to be true substrate of pyruvate kinase which presumably was
utilized by hepatocytes stimulated by nitroimidazole. It further suggested the binding of
ADP to unbound and fully bound enzyme conformations involving different groups within
active site. The binding of ADP and ADP free conformation of enzyme did not involve
groups that ionized within applied pH range in nitroimidazole and progesterone stimulated
hepatocytes but binding of ADP to the fully bound enzyme conformation (cooperativity)
involved the ionizable groups with high pK values (at pH 6.6-7.4). Moreover, phosphoenol
pyruvate bound enzyme ionization has been previously reported to promote cooperative
binding of ADP in diseased cell. In present study, ADP binding becomes less cooperative by
stimulated hepatocytes. It is possible that enzyme may have two active conformations. So,
allosteric interactions actually might have altered the mode of ADP binding in normal cells
while they were non-effective in pyruvate kinase enzyme of stimulated hepatocytes.
Pyruvate kinase active site conformation perhaps seems to change the ADP binding.
Fig. 8a. The pyruvate kinase enzyme activity in control, amoebic liver abscess and
nitroimidazole treated hepatocytes is shown in presence of different enzyme regulatory
additives (A-I).
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
Fig. 8b. The pyruvate kinase enzyme activity in control, amoebic liver abscess and
nitroimidazole treated hepatocytes is shown in presence of different enzyme regulatory
ADP additives (2-5 mM ADP).
Fig. 8c. The pyruvate kinase enzyme activity in control, amoebic liver abscess and
progesterone treated liver cells is shown in presence of different enzyme regulatory
additives (ATP, PEP+FDP,PEP inhibitors).
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Enzyme Inhibition and Bioapplications
Fig. 9. The effect of nitroimidazole on phosphofructokinase enzyme activity of isolated
hepatocytes from control, amebiasis and nitroimidazole groups. Hepatocytes from each
group were sedimented and homogenized in Kreb Hanseleit buffer pH 8.0 containing 2.5
mM dithiothretiol, 0.1 mM EDTA and 50 mM NaF for 90 seconds followed by centrifugation
at 27000 g for 10 minutes. One unit enzyme activity catalyzed 1 micromole fructose 6
phosphate to fructose 1,6 DiP per minute per mg enzyme protein.
4.3 Phosphofructokinase
Phosphofructokinase enzyme regulation in isolated hepatocytes cultures added with
amoebic trophozoites, nitroimidazole and progesterone exhibited specific regulatory
behavior. Addition of fructose 1,6 Diphosphate additive exhibited a stimulation of
phosphofructokinase activity (see Figure 9). The effect of progesterone was seen as best
response at 10 nM in hepatocytes of all groups while fructose 1,6 diphosphate showed
maximum effect at concentration of 10 nM on the hepatocytes in presence of trophozoites
added in hepatocyte cultures. However, hepatocyte cultures in other groups did not show
any alteration (see Figure 9). Similarly, progesterone addition to hepatocytes in presence of
trophozoites exhibited maximum fructose 1,6 diphosphate in the medium. The addition of
progesterone concentration was within the physiological range (see Figure 9). Progesterone
was presumed to effect enzyme activity by influencing intracellular level of various
allosteric effectors of enzyme. Fructose 1,6 diphosphate acts as potent activator of enzyme
activity. The phosphofructkinase enzyme activity was reported as cyclic AMP dependent
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
217
[Pilkis et al.1979, Sanchez-Martinez et al.2000]. However, nitroimidazole added hepatocytes
did not respond or responded slow after progesterone addition.
4.4 Phosphodiesterase
Cyclic AMP dependent phosphodiesterase regulation over prostaglandin E2 and F2
synthesis was stimulated in Kupffer cells added with trophozoits and nitroimidazole.
Phosphodiesterase activity in Kupffer cells was dependent upon calcium ion concentration
which was regulatory factor for cyclic AMP pool and prostaglandin production. In amoebic
trophozoite added cultures, cells exhibited insignificant elevated enzyme activity while
nitroimidazole addition did not alter the enzyme activity as shown in Figure 10. The
enzyme behavior was similar in nature for its dependence on cyclic AMP secretion,
prostaglandin synthesis and calcium ion. However, tropozoites stimulated more cyclic AMP
secretion and prostaglandins. It provides the probable mechanism of trophozoite stimulated
macrophagal action. Earlier calmodulin dependent adenylate cyclase and phosphodiesterase
regulation for cyclic AMP pool have been described [Hidi et al.2000]. Moreover, the
stimulated prostaglandin synthesis was described as calmodulin dependent cyclic AMP
regulated mechanism [Hidi et al.2000]. Thus it seems that amoebic trophozoite addition
with hepatocytes regulates intracellular calcium ion concentration which controls
prostaglandin synthesis via phospholipid and arachidonic acid precursors in hepatocytes.
Thus synthesized prostaglandins regulate the cyclic AMP intracellular levels. The similar
mechanism has been described as “self-limiting” mechanism [Martina et al.2006].
Fig. 10. Cyclic AMP dependent phosphodisesterase activity and secretion of prostaglandin E2
and F2 in isolated kupffer cells from different groups. The phosphodiesterase enzyme activity
in amebic liver abscess group showed highest enzyme activity after additives were added.
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Enzyme Inhibition and Bioapplications
4.5 Enzyme inhibition in respiratory burst of liver cells
An initial evidence of liver cell survival at different oxygen pressures is shown in Figure 11
to highlight respiratory burst of enzymes. The inhibited enzymes related with cell
respiration are presumably considered responsible of induced respiratory burst (hypoxia) in
liver cells. Respiratory burst process including enzyme alterations (superoxide dismutase,
glutathione reductase, NADPH oxidase enzyme activity) was suggested as initial Kupffer
cell stimulation or respiratory burst by additives and followed by energy and oxygen
depletion as possible cause of hypoxia and cell viability loss as important findings in
following section.
Fig. 11. Effect of oxygen supply (shown in different pressures) to liver cells. Notice the low
oxygen supply depletes the oxygen content in cells while high oxygen supply keeps cells in
in good survival. For long survival, reoxygenation of cells is needed(see green vertical line).
4.6 Superoxide dismutase enzyme activity in respiratory burst of Kupffer cells
The superoxide dismutase enzyme activity in isolated cells was comparable and showed
minimal difference in both control and additive triggered Kupffer cells. Addition of sodium
cyanide concentration showed small effect on elevated superoxide dismutase activities in
both control and nitroimidazole triggered Kupffer cells. However, Kupffer cells showed
elevated respiratory burst activity significantly high after adding 10 nM sodium cyanide in
the cell culture medium as shown in Figure 12. However, the response of Kupffer cells with
sodium cyanide addition was more in trophozoites added cell cultures at 10 mM sodium
cyanide concentration. Zymosan stimulation on superoxide production was insignificantly
high in the Kupffer cells in both groups. Earlier stimulation of zymosan induced superoxide
anion production in Kupffer cells by sodium cyanide was reported to show the effect of
cyanides on cytosolic superoxide dismutase enzyme activity [D'Alessandro et al.2011,
Shahhosseini et al.2006]. Moreover, zymosan stimulated superoxide production was
reported in Kupffer cells as reaction of cytochrome C reduction [Duan et al.2004].
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
219
Fig. 12. The enzyme activity was estimated in Kupffer cells cultures added with sheep
erythrocytes, trophozoites, nitroimidazole. The Kupffer cells were maintained for 48 hours
or more to determine superoxide anion production in presence of 1.0 mM and 1.0 mM
sodium cyanide additive added in culture medium. In brief, 3 x 105 Kupffer cells were
suspended in Kreb’s Henseleit buffer containing 5 mM glucose, 3 mM calcium chloride pH
7.4 in presence of 50 M ferricytochrome in total 1.0 ml reaction mixture and cells were
incubated for 15 minutes in presence of different sodium cyanide additives. The reaction
was started by addition of zymosan 500 g per ml to the medium after 10 minutes at 37 C
supernatant was taken, centrifuged at 8000 x g in cold for two minutes and assayed for
cytochrome C reductase enzyme activity. Ref. Sharma, R. et al. 2010.
The O2 superoxide anion radical production as result of respiratory burst activity showed
proportionate elevated enzyme activity in isolated Kupffer cells from cultures added with
trophozoites and nitroimidazole additives. The active role of reactive oxygen species and
superoxide dismutase activity in cultured Kupffer cell damage during hypoxia is shown in
Figure 12. Previous study supported these observations on cellular damage and cytotoxicity
from reactive oxygen species from superoxide anion acting as a scavenger [Sharma et
al.2010]. Another possibility of Kupffer cell toxicity by additives may be associated with
cytokines, specifically TNF-α which was associated with the cytotoxicity [Sharma et al.2007].
Previous study also reported a decreased hepatocyte cell viability associated with altered
cAMP phosphodiesterase and HMP shunt activity as a result of hypoxia [Carre et al.2010].
Authors suggested that the oxygen insufficiency is possibly a consequence of energy
depletion and increased oxygen reactive species [Keeling et al.1982]. Other reports support
this view of vasodilatory sensitive ATP depletion in Kupffer cells [Ana et al.2011]. The
reactive oxygen species production also seems as another cause of hypoxia as suggested by
220
Enzyme Inhibition and Bioapplications
the nature of superoxide dismutase as additive cyanide sensitive enzyme. We observed a
good reason of association between superoxide anion scavenger, glutathione reductase and
NADPH oxidase as cause of cell viability loss leading further to Kupffer cell damage. In this
direction, ample evidence supports the possibility of activation of HMP shunt due to
depleted ATP and NADH in the Kupffer cells [Grahm, 2000; Spolarics et al. 1991].
4.7 HMP shunt pathway in respiratory burst
Hexose monophosphate shunt activation during respiratory burst in isolated Kupffer cells
exposed with amoebic trophozoites and nitroimidazole showed enhanced enzyme activities
as shown in Figures 13A-13C. HMP shunt activation is measured in three ways: 1. HMP
shunt activity in presence of additives such as latex, sheep erythrocytes, trophozoites; 2.
HMP shunt activity as radiolabeled glucose conversion to carbon dioxide; 3. HMP shunt
activity as enzyme inhibition.
a.
b.
c.
The HMP shunt activity was maximum and it elevated in Kupffer cells after incubation
with sheep erythrocytes and trophozoites while Kupffer cells exposed with inert latex
particles did not show any activation or change in HMP shunt activity. Sheep erythrocytes
and trophozoites both stimulated glucose metabolism via hexose monophosphate shunt
activity. Perhaps latex particles being biologically inert nature could not change the
cellular activity at all. So, latex particles were phagocytosed without altered HMP shunt
activity. It suggested that HMP shunt activity may be a triggering event in Kupffer cell
activation dependent not solely upon intracellular fate of ingested material or particles.
However, other additives such as zymosan, erythrocytes and trophozoites also showed
same response. Previous study reported similar response of zymosan stimulated and
iodoacetate inhibited Kupffer cell glucose-6-phosphate dehydrogenase and
phosphogluconate dehydrogenase enzymes.[Sharma et al.2010]
The HMP shunt activity as [14C] glucose conversion to [14C]-CO2 liberated product
indicated the active inflammation elicited by macrophage response of Kupffer cells
[Schorlemmer et al. 1999]. The HMP shunt activity in control and triggered Kupffer
cells was measured by [14C]CO2 release in dpm from labeled [14C] glucose per minute
per mg protein in presence of different effectors known as quenching the hexose
monophosphates in glycolysis. The HMP shunt activity was similar with no difference
(P value > 0.001) in control compared with triggered Kuffer cells with effector and inert
latex particles did not alter the HMP shunt activity. The HMP shunt activity was
significantly enhanced in triggered Kupffer cells in presence of trophozoites (P value <
0.001) but the HMP shunt enhancement was lesser in presence of erythrocytes. It
indicated the dependance of HMP shunt activity on nature of effector either inert or
surface active. The latex was inert, erythrocytes were nonvirulent and trophozoits were
virulent as a result HMP shunt activity was enhanced in the order of latex <
erythrocytes < trophozoits added in the medium of Kupffer cells.
The trophozoites further activated the Kupffer cell glucose-6-phosphate and
phosphogluconate dehydrogenase enzymes in association of HMP shunt activity. In
following section, enzyme inhibition in liver cells is reported in control, hypoxic cells in
different groups. Hexose monophosphate shunt activation was enhanced during
respiratory burst in isolated Kupffer cells in amoebic trophozoits and nitroimidazole
added cultures. In the cultures added with trophozoits and nitroimidazole both,
Kupffer cells exhibited significantly elevated HMP shunt activation. The HMP shunt
Mechanisms of Hepatocellular Dysfunction
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
221
activity was highly elevated in all groups cultures incubated with sheep erythrocytes
and amoebic trophozoits while cells with latex particles did not show any activation.
Perhaps latex particles being biologically inert could not change the cellular sinusoidal
function activity. Sheep erythrocytes and trophozoits perhaps stimulated the glucose
metabolism via hexose monophosphate pathway but latex particles were phagocytosed
without altered shunt activity. It suggested that HMP shunt may be a triggering event
in Kupffer cell activation dependent not upon intracelllular fate of ingested material or
particles [Knook et al. 1980]. The trophozoits added Kupffer cells showed direct
interaction
with
increased
glucose-6-phosphate
dehydrogenase
and
6phosphogluconate dehydrogenase enzyme activities as described as a result of
enhanced hexose monophosphate shunt activity. Earlier zymosan stimulated and
iodoacetate additives inhibited Kupffer cell glucose 6 phosphate dehydrogenase and 6phosphogluconate dehydrogenase activities have been reported [Heuff et al. 1994].
Inhibition of Glutathione Reductase enzyme activity in respiratory burst: The
glutathione reductase GSH specific enzyme activity was measured in hypoxic
hepatocytes in presence of different GSH enzyme inhibitors such as DEM, antimycin,
BSO as additive effectors. To demonstrate the dependence of enzyme activity on
exposure time of inhibitors, the hypoxic hepatocytes were incubated in presence of
effectors and enzyme activity was measured in medium. At 0 hour, the enzyme specific
activity was same in hepatocytes in all groups. After 24 hours, the activity in
hepatocytes was inhibited maximum by actinomycin while DEM and BSO inhibited the
enzyme at similar extent and hypoxic hepatocytes exhibited no difference in enzyme
activities (P value > 0.001) (see Figure 14). After 48 hours, the GSH enzyme activity was
inhibited maximum by addition of BSO and minimum by antimycin in cell cultures.
However, the behavior pattern of enzyme inhibition by DEM and BSO was similar.
After 72 hours, the inhibition pattern of GSH enzyme activitity by inhibitors was same
as after 48 hours. After 96 hours, all inhibitors showed maximal inhibition and hypoxic
hepatocytes showed same specific activities in all groups. Antimycin inhibitor exhibited
lesser inhibition activity of hypoxic GSH enzyme initially during 24-48 hours.
Inhibition of NADH oxidase in respiratory burst: The respiratory burst activity was
described as increased activity in liver macrophages during hepatic cell damage
showing increased oxygen consumption, stimulation of HMP shunt and elevated
superoxide production with activated membrane NADPH oxidase enzyme as one of the
mode of phagocytosis [Dusting et al. 2005]. Moreover, our experiments suggested the
specific roles of additives on respiratory burst biomarkers: 1. The increased sodium
cyanide concentration showed stimulation of superoxide dismutase activity; 2. The live
erythrocytes and trophozoites showed HMP shunt stimulation while inert latex
particles showed no effect on HMP shunt activity suggesting the association of additive
nature as determinant of the HMP shunt activity or trio-enzyme inhibition; 3. The
glutathione plays a critical role in surviving Kupffer cells during hypoxia and indicates
the stimulated reactive oxygen species in respiratory burst activity [Bhatnagar et
al.1981]. All these respiratory burst biomarker activities showed dependence on the
nature of additives in corroboration with earlier reports [Kumar et al.2002; Forman et al.
2002; Ding et al. 1988]. In present chapter, both HMP shunt activity and respiratory
burst activity (superoxide dismutase, glutathione reductase, NADPH oxidase enzyme
activity) are outlines for suggested initial Kupffer cell stimulation by additives and later
followed by energy and oxygen depletion as possible cause of hypoxia and cell viability
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Enzyme Inhibition and Bioapplications
loss as important finding. However, association of Kupffer cell stimulation and hypoxia
is not conclusive and needs further investigation.
Fig. 13. Hexose monophosphate shunt activity is shown in isolated Kupffer cells from ameba
infected and nitroimidazole treated cells in presence of erythrocytes, trophozoits and latex
additives (see panel on top). In 3.0 ml medium, 1 micro Curie 14C glucose was added and
simultaneously particles added with Kupffer cells in ratio 5:1(particle:cell) for erythrocytes, 2:1
for amebic trophozoits and 40:1 for latex. After one hour, 2 ml of cell free medium soaked by
filter paper wetted with 100 µL of 10% KOH in counter well which further was added with 200
µL 1.0 N H2SO4 and after 15 minutes filter paper strip elute in tritosol were measured for
scientillation counts. The trophozoits showed maximum activity while inert latex beads did
not show any change in activity(see panel at bottom).Ref. Sharma, R. et al. 2010.
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
GSH activity (IU)
30
223
0 hour
24 hours
48 hours
72 hours
96 hours
20
10
O
M
m
1


M
1
an
t im
m
M
yc
i
D
n
BS
A
EM
0
Fig. 14. Effect of added stimulators on Glutathione reductase enzyme activity in hypoxic
hepatocytes in culture. Ref. Sharma, R. et al. 2010.
4.8 Cytochrome oxidase
Cytochrome C oxidase was determined based upon oxidation of ferrocytochrome C by
decrease in absorbance at 550 nm on spectrophotometer. In 3 ml reaction mixture containing
66 mM potassium phosphate buffer pH 7.4. 20µM reduced cytochrome C and 20µl
mitochondrial protein reaction was followed at 30°C for two minutes. Reading was recorded
after two minutes for absorbance of completely oxidised cytochrome C by adding 0.05 M
potassium ferricyanide. The activity of cytochrome C oxidase was expressed in n moles of
cytochrome C oxidase/mg protein/min which is equal to concentration of cytochrome C x
K divided by concentration of protein. For calculation of cytochrome C concentration a
millimolar extinction coefficient of 19.1 was employed for reduced vs oxidized cytochrome
C at 550 nm as described by Emmelot et al.1964.
4.9 Inhibition of NADPH cytochrome P450 reductase
Cytochrome P 450: Cytochrome P 450 was measured by spectrophotometer in absence or
presence of carbon monoxide and determined by the method [Guengerich et al.2009]. In
brief, 0.1 ml microsomal fraction in two cuvettes test and standard which mere added 1,5
mg sodium dithionite and carbon monoxide passed in only test sample cuvette for 25
seconds and same repeated again and ΔA recorded at 450 nm for cytochrome P 450 content
in n moles per mg protein calculated by extinction coefficient 91 mM/cm path length.
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Enzyme Inhibition and Bioapplications
4.10 Inhibition of adenylate cyclase
Adenylate cyclase regulatory properties: Adenylate cyclase enzyme in isolated hepatocytes has
been reported as tool of regulatory studies of hormones, enzymes in the presence of Gs
bound receptors [Small et al. 2000]. Membrane adenylate cyclase activity is defined due to
its multi-pass transmembrane protein made of two cytosolic segments as shown in Figure
15. These segments play active roles in association of Gsα-GTP. The GTP induced Gsα
binding with adenylate cyclase can be a key mechanism of adenylate cyclase stimulation or
inhibition during progesterone bound α-adrenergic receptor [Sunahara et al. 1997]. The
possible mechanism of adenylate cyclase activation or inhibition by progesterone is shown
in Figure 15. The membrane adenylate cyclase activity can be an indicator of membrane
viability. However, the enzyme activity depends on the type and nature of effecter. The
behavior of liver cells in culture in the presence of effectors may indicate the regulatory
effect of stimulators in vivo. However, in cultures, hepatocytes and Kupffer cells may likely
experience different physiological environment different from in vivo. In controlled liver cell
culture conditions of medium, the liver cells showed adenylate cyclase stimulation in a
specific manner by effectors such as progesterone, nitroimidazole, GTP, GITP, and
prostaglandins [Aoshiba et al. 1997]. The receptors for prostaglandins interact with Gsα
subunits. Gsα-GTP complex binds with α-adrenalgic receptor made of seven subunits as
shown in Figure 5. Binding of receptor with Gsα-GTP protein inhibits adenylate cyclase
activity and cAMP formation. The present study highlights the behavior of liver cells and
the characteristic features of their cyclic AMP dependent adenylate cyclase regulatory
system in culture.
Progesterone modulation of adenylate cyclase activity: Progesterone modulates the activity of
adenylate cyclase and adenylate cyclase response is hormone sensitive [Gilman et al.
1984]. The progesterone stimulates the receptor activating the adenylate cyclase by
stimulatory Gsα-GTP complex resulting with formation of cAMP. Hepatocytes with
progesterone added in medium exhibited the enhanced enzyme activity. The addition of
GTP stimulator did not change the adenylate cyclase activity. In contrast, GITP stimulated
cells exhibited an apparent adenylate cyclase activation. Unlike the adenylate cyclase
enzyme activity in the presence of different effectors in stimulated hepatocytes, the rates
of cyclic AMP production was delayed in control hepatocytes. However, progesterone
addition to hepatocytes did not exhibit significant delay in GITP stimulated adenylate
cyclase activity. Thus reason can be speculated that progesterone binds with stimulatory
receptors on hepatocytes which lead to increase in intracellular cyclic AMP accumulation
through adenylate cyclase activation via guinine nucleotide regulated mechanism [CohenTannoudji et al. 1991]. The Kupffer cells also showed similar guanine regulatory enzyme
properties.
The progesterone response of adenylate cyclase activity is a two phase process [Ko, et
al.1999; Martin et al. 1987]. First, progesterone receptor and catalytic subunit (C complex) of
adenylate cyclase occupy the place on membrane followed by Gsα protein interaction with
enzyme to make G protein C complex [Ko et al. 1999]. However, the addition of GITP
stimulated the adenylate cyclase to a greater extent in hepatocytes more than pre-exposed
hepatocytes to progesterone. Recently, G protein-enzyme complex formation by GTP/GITP
stimulation has been reviewed [Niu et al. 2003].
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
225
Fig. 15. Adenylate cyclase enzyme activity is shown in control, nitroimidazole, progesterone
and amoebic liver abscess recruited liver cells in presence of GITP/GTP stimulators,
protein* additives. Ref. Sharma, R. et al. 2010.
Nitroimidazole stimulation of adenylate cyclase activity: Nitroimidazole is recently emerged as
anti-tumor and radiosensitizer compound. It is used in tumor imaging, therapy and antiamoebic applications. Nitroimidazole has phenolic rings and phenolic side chains as shown
in Figure 6. So, it is obvious that the side chains interfere with the dephosphorylation of ATP
by phosphodiesterase enzyme. It appears to result with inhibition of hepatocyte adenylate
cyclase catalytic subunit function in the presence of nitroimidazole. The cytotoxicity of
nitroimidazole is poorly studied and reported. However, its anti-amoebic effect has been
proven to decrease hepatic and intestinal infection rate. The nitroimidazole is widely
reported as single dose in treatment of amoebic liver abscess. Moreover, nitroimidazole
induced cytotoxic effect on human hepatocytes are still not known. DNA strand breakage,
226
Enzyme Inhibition and Bioapplications
energy regulatory enzymes, phagocytosis, and immune response have been reported as
nitroimidazole induced cytotoxic effects in animals [Zheng et al. 1997; Whitaker et al. 1992;
Singh et al. 1991]. The role of nitroimidzole is understood to restore energy regulatory
process involving adenylate cyclase, phosphodiesterase, cAMP formation in hepatocytes
and reported as hormone and GTP specific in action. The nitroimidazole derivatives
inhibited anterior pituitary cell function apparently by their direct effect on the catalytic
subunit of the adenylate cyclase holoenzyme [Stalla et al. 1989]. The nonparenchymal Kuffer
cells play a significant role of phagocytosis sensitive to prostaglandins. Recently,
nitroimidazole derivatives have been identified as anticancer potential agents. Still the role
of imidazoles as anticancer agent is not established [Anderson et al. 2006]. The indicated
study the association of imidazole ring structure with the cyclic AMP dependent adenylate
cyclase catalytic subunit regulatory mechanism in liver cells.
5. Inhibition of drug metabolizing enzymes in isolated liver cells
Drug metabolizing enzymes are abundant in microsomes and lysosomes. Most of these
enzymes are elevated or secreted at the sites of inflammation, disease conditions and in
presence of foreign bodies. An elevated enzyme in cells and secretion from lysosomes or
microscomes action is in defense by ‘suicide action’ of microbody against disease or foreign
bodies. Drugs inhibit these enzymes or bring back any disease or inflammation induced
effect on enzyme activity in liver cell. Characteristic inhibition of enzyme activities in
different groups by nitroimidazole are shown in Figure 15. Recently, we reported the
enzyme inhibition by nitroimidazole in detail [Sharma 2011a, Sharma 2011b]. In general,
lysosomal enzymes are peptidases, act at acidic pH, and participate as terminal scavengers
of harmful anions, radicals and peptides. For details, read section 2.
Nitroimidazole inhibits the liver enzymes by making adduct for glyoxyl-glutamine
metabolism in liver cells as author described in detail [Sharma, et al. 2011].
6. Phagocytosis
Although the clearance and distribution of ligand molecules in circulation represent the
function of hepatic sinusoidal cells, these mechanisms reveal a network that is more intricate
than would at first seem, since several receptors are common to not only one type of cell, but
also to two or three types of cells in the liver. In the case of latex particles in which their
uptake by a particular cell type seems to be determined by their size, sinusoidal endothelial
cells are able to internalize particles up to 0.23 microns under physiologic conditions, in
vivo, and larger particles are taken up by Kupffer cells. However, when the phagocytic
function of Kupffer cells were impaired by frog virus or alcohol, the endothelial cells were
reported to take up particles larger than 1 micron in diameter after the injection of an excess
amount of latex particles. Endothelial cells would thus constitute a second line of defense in
the liver in that they remove foreign materials from the blood when Kupffer cell phagocytic
function is totally disturbed [Schoremmer et al. 1990]. This potential role may not, however,
be fully expressed under physiologic conditions when Kupffer cells are active in clearing
foreign substance from the circulation. The functions of liver sinusoidal cells are varied and
complex and these cells can be regarded as "a sinusoidal cell unit." This cellular interaction
must be taken into account for any quantitative analysis [Kampas et al. 1999].
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
227
Fig. 16. Lysosomal and microsomal enzymes in isolated nonparenchymal liver cells are
shown in control, nitroimidazole, progesterone, amoebic liver abscess recruited livers. Note
inhibitory action of nitroimidazole on liver cells to bring normal activities in comparison to
amoebic liver abcess recruited cells. Ref. Sharma, R. et al. 2009.
228


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Enzyme Inhibition and Bioapplications
Enzyme inhibition in phagocytosis: In our experiments, liver cell cultures were
mixed with zymosan (7:1 particle cell ratio); erythrocytes (5:1 particle cell ratio);
amebic trophozoits (2:1 particle ratio), latex (20:1 particle cell ratio) and
nitroimidazole was added 1 µM/106 cells. Kupffer cell phagocytic activation in
isolated Kupffer cells from human livers (added with amoebic trophozoits) was
elevated 2-3 fold over normal Kupffer cells. Kupffer cells incubated with zymosan
and trophozoits exhibited maximum phagocytic activity while with latex and sheep
erythrocytes these exhibited minimum and insignificantly increased activity
respectively. Increased phagocytic activity was time dependent which was exhibited
by -glucuronidase, lactate dehydrogenase and plasminogen activator activity.
Phagocytosis of zymosan and sheep erythrocytes triggered the immediate release of 
glucuronidase, stimulated synthesis of cellular lactate dehydrogenase and induced
delayed production with secretion of plasminogen activator. Inhibition of these
enzymes as nitroimidazole response is shown in Figures 16-18, in detail. Major
findings were: 1. effect of nitroimidazole and latex were minimum. Similar
observations were reported in earlier study [Kamps et al. 1999, Bhatnagar et al. 1981,
Kuttner et al. 1982]; 2. nitroimidazole was biotransformed into water soluble products
and cleared by drug metabolizing enzyme system of Kupffer cells; 3. latex particles
being inert confirmed the independence of Kupffer cell activation from intracellular
fate of ingested material; 4. amoebic trophozoits with their toxins or specific
membrane surface signals stimulate Kupffer cells for phagocytosis resulting early
release of lysosomal hydrolases than the release by other particles [Virk et al 1989,
Virk et al. 1988]. However, increased lysosomal enzymes in Kupffer cells incubated
with amoebic trophozoits were reported as a consequence of phagocytic activation
[Froh et al. 2003]. Recently, these phagocytic activation changes due to nitroimidazole
have been reported as associated with prostaglandin synthesis and their release
[Spolarics et al. 1997].
The glucuronidase activity showed the enhanced enzyme activity trend in all additives
but different enzyme enhancement for different additives. In control cells, the activity
enhanced during 24-36 hours in incubation. The erythrocytes showed most
vulnerability to Kupffer cells in initial 24 hours. Zymosan beads showed slow response
to phagocytosis while latex remained inert to Kupffer cells. The nitroimidazole showed
normal enzyme pattern. It was interesting that maximum rate of phagocytosis was
observed during 24 and 36 hours except erythrocytes.
Lactate dehydrogenase enzyme activity showed the enhanced enzyme activity trend in
all additives except latex additive but different enzyme enhancement for different
additives. In control cells, the activity enhanced during 12-36 hours in incubation. The
erythrocytes showed most vulnerability to Kupffer cells in initial 24 hours. Zymosan
beads showed maximum response to phagocytosis while latex remained inert to
Kupffer cells. The nitroimidazole showed inhibitory enzyme activity in Kupffer cells
but after 24 hours the response was stimulatory to enzyme activity (delayed enhanced
enzyme stimulation as a result phagocytosis). It was interesting that maximum rate of
phagocytosis was observed during 48 hours. However, the leaked out LDH activity
represented the probable phagocytic activity of Kupffer cells relationship with time.
Nitroimidazole addition in incubation medium perhaps showed the minimizing
amoebic virulence [Sharma 2010]. Earlier lysosomal enzyme release with increased
LDH cellular levels has been reported [Koppele et al. 1991].
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
229
Fig. 17. Phagocytic activity of Kupffer cells is shown as β glucuronidase enzyme activity of
cells after adding zymosan, erythrocytes, entamoeba histolytica trophozoits, latex beads
additives, and nitroimidazole as drug in cultures of Kupffer cells from liver biopsy samples.
The enzyme activity was measured after 1, 2, 3, 4 days of incubation. Each point value
represents the average of 3 readings of observations. Ref Sharma,R. et al. 2009.
Fig. 18. Phagocytic activity of Kupffer cells is shown as lactate dehydrogenase enzyme
activity of cells after adding zymosan, erythrocytes, entamoeba histolytica trophozoits, latex
beads additives, and nitroimidazole as drug in cultures of Kupffer cells from liver biopsy
samples. The enzyme activity was measured after 1, 2, 3, 4 days of incubation. Each point
value represents the average of 3 readings of observations. Ref Sharma,R. et al. 2009.
230

Enzyme Inhibition and Bioapplications
The plasminogen activator induced lysis of Kupffer cells showed the enhanced
plasminogen activity only after 36 hours. Initially the activity trend in all additives was
same and immune to any additive added. In control cells, the plasminogen activity
remained unaltered while kupffer cells showed immunity to zymosan and latex additives
throughout the incubation period. However Kupffer cells showed sharp enhancement of
plasminogen activity after 36 hours during 36-48 hours in incubation. The trophozoits
showed maximum effect on plasminogen activator in Kupffer cells after 36 hours.
Plasminogen activator induction has been reported dependent on de novo enzyme
synthesis in stimulated macrophage by endotoxins and asbestos [Nagaoka et al. 2003].
Fig. 19. Phagocytic activity of Kupffer cells is shown as plasminogen activator activity of cells
after adding zymosan, erythrocytes, entamoeba histolytica trophozoits, latex beads additives,
and nitroimidazole as drug in cultures of Kupffer cells from liver biopsy samples. The enzyme
activity was measured after 1, 2, 3, 4 days of incubation. Each point value represents the
average of 3 readings of observations. Each point on graph represents the activity as extent of
cell lysis in percent increase over basal value of 100. Ref Sharma et al. 2009.
7. DNA synthesis in Kupffer cells
Stimulated DNA synthesis is considered as defense mode. From physiological point of view,
it is true that drastic change in enzymatic pattern in liver regeneration permits DNA
synthesis. Major enzyme players are DNA polymerase and DNase (allosteric complex
enzyme) in mitosis. We present five sequence steps of events: deoxyribonucleotides,
ribonucleotides, DNA, RNA and proteins (see Figure 20) [Bresnick et al.1970].

DNase exposes 3’-OH end groups thus increases capacity of chromosomal DNA in
vivo. DNA polymerase plays a passive role in DNA synthesis in presence of high
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration


231
chromosomal DNA and deoxyribonucleotides. Priming DNase action on native and
denatured DNA initiates DNA synthesis. Earlier study reported that arginase enzyme
acts strong DNA synthase inhibitor perhaps due to lack of neosynthesized histone
(arginine-rich histone) by reducing priming state of chromosomal DNA. DNA
sysnthesis by means of additive or replicative DNA polymerase needs 3’-OH roup and
polymerization proceeds from 5’-phosphate end to the growing 3’-OH end.
Increased RNA synthesis also increases ribonucleotide pool, rise in RNA polymerase
activity, increase in polyamine synthesis and s-RNA methylation. The temporal
relationship between kinetic changes in the DNA channel and the RNA channel seem to
indicate that RNA channel is primer of DNA channel. In other words, cyclic AMP level
may initiate both DNA and RNA synthesis beginning at the nucleotide level under
hormonal regulation. Best example of elevated RNA sysnthesis in liver regeneration is
elevated orotate pyrophosphorylase/decarboxylase to keep CTP/UTP pool in presence of
UTP/CDP coenzymes for glycogenesis and lipogenesis respectively. UMP stimulated
orotate-tRNA labeling around 18s-28s RNA complex step is preceded by an increase in
tRNA methylation by tRNA Methylase enzyme. For its action, short chain of
monocistronic mRNA on 18s component acts as primer for synthesis of constitutive RNA
and export protein synthesis. Nobel Prize was awarded in 2009 on resolving the
intricacies of ribosomal subunit interplay and consequent role of DNA/RNA Polymerase
enzyme reactions in protein synthesis during drug induced regeneration [Schmeing et
al.2009]. However, other enzymes RNase activity, cataionic polyamine play role in
stabilization of ribosomal RNA (labeled methionine to spermidine) in protein synthesis.
Increased polymine synthesis enhances ornithine decarboxylase to increase amino acid
pool in presence of cAMP induced prostaglandins mediated growth hormone. Moreover,
growth hormone stimulates thymidine kinase in liver. Author describes DNA synthesis in
isolated Kupffer cells during liver regeneration after nitroimidazole treatment in amoebic
liver abscess in following section.
Kupffer cells isolated from human amoebic liver abscess liver-biopsies and progesterone
treated Kupffer cell in cultures were induced for DNA synthesis by adding
nitroimidazole. This offered a simple model to study the mechanism of cell DNA
induction and interdependence between two cell populations. Earlier such interactions
between cells were reported regulated by thymidine kinase (thymidine pathway) and
ribonucleotide reductase (thymidylate pathway)[Bresnick et al.1970]. Conceptually,
immediately after disease in regeneration, liver cell gets confronted a challenge of
increased protein synthesis for cell division involving metabolic enzymes leading to DNA
synthesis (binding of ribonucleotides with deoxyribonucleotides). Two pathways play
role in this process by Thymidine pathway and thymidylate pathway. Thymidylate
pathway for DNA synthesis needs dTTP supply, thymidine kinase, TDP kinase to keep
TMP pool. Basically, a repressor molecule (thymidine) inhibits transcription of thymidine
kinase by allosteric regulation. Thymidine kinase is a TTP insensitive dimeric form (one
catalytic site and other allosteric site) means Km for enzyme decreases with elevated ATP
concentration (ATP positive effector and TTP negative effector). dimeric form and shows
sigmoid behavior of thymidine kinase [Barroso et al. 2005]. Activation of thymidine kinase
also depends on cyclic AMP activated protein kinase. On other side, ribonucleotise
reductase reduces CDP in presence of dCMP deaminase to make dUMP (a substrate for
thymidylate synthesis) by thymidylate sysnthetase in presence of N5,N10 hydroxymethylFH4. dCMP is good enzyme inhibitor of thymidylate sysnthetase and activator of folate
reductase to keep net high TMP pool. Ribonucleotide synthesis is regulated by
232

Enzyme Inhibition and Bioapplications
ribonucleotide reductase. Increase in CTP build up dCTP, purine deoxyribonucleotides
and TTP pools[Cheung et al.1970].
Several types of growth factors have been reported as regulatory substances participating
in liver regeneration process. Stimulation of DNA synthesis in Kupffer cells by amoeba
trophozoites is shown here as response induced by colony stimulating factor. Since
measured increase of radio label uptake by increasing amoeba trophozoite concentration
in medium reflected an increase of cell number entering into S phase rather than higher
rate of [3H] TdR incorporation seen in all the cells, it was possible that different Kupffer
cells normal or pretreated with amoeba (from these different liver cell populations) may
vary in susceptibility to colony stimulating factor (CSF). High concentrations of amoeba
caused a reduction of thymidine uptake and remarkably reduction was measured on
whole cover slip scintillation counts. It reflected lower incorporation of [3H] TdR into each
DNA synthesizing cells. This phenomenon may be due to reduced capacity of Kupffer
cells uptake as a result of DNA synthesis but it requires further inhibitor study or
presence of CSF. Thus higher concentration of amoebae trophozoites caused a reduced
Kupffer cell uptake capacity during CSF production.
Fig. 20. DNA synthesis in liver cells in culture medium is shown in presence of macrophage
growth factor measured as cpm/flask and cpm/cover slip during 4-10 days. Notice the
nitroimidazole induced inhibition of DNA synthesis in amebic treated cells.Source: Sharma,
R.(1990) Ph.D dissertation.
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
233
8. Inhibition of liver lysosomal enzymes in protein metaboilism
Isolated lysosomal preparations from L[1- 1 4 C] leucine (25 Ci per 100 gm wt)
intraperitonially injected human Kupffer cell cultures exhibited percent degradation at pH
3.0, 4.0,5.0, 6.0,7.0, 8.0 and 9.0 at temperatures 10 °C, 20 °C, 30 °C, 40 °C, 50 °C and 60 °C, in
presence of amoebic trophozoits after 15 minutes(short lived) and 15 hours(long-lived)
addition as shown in Figures 21. Major players in this liver regeneration process are
ribosomes (sitting on endoplasmic reticulum), GTP (ribotide pool) and coenzymes for
ribotide polymerase enzymes (a balance in thymidine kinase and thymidylate synthetase).
Other enzyme inhibition is LDH-M type gets converted to LDH-H type or lactate 
pyruvate (accelerated glycolysis). In simultaneous other set of experiment, Kupffer cell
lysosomes in presence of leupeptin, pepstatin inhibitors and nitroimidazole in 100 g, 100
g and 50 mM per ml concentration respectively showed aryl sulphatase enzyme activity
changes (see Figures 21,22) in the presence of 0.3 M sucrose at 37 °C for 60 minutes at pH 5.5
in incubation mixture of Kupffer cell lysosomes and additives. Proteolysis rate and
degradation of short and long lived proteins in isolated lysosomes from the Kupffer cell
Fig. 21. Figure shows the effect of of radiolabeled [14C]-Leucine in Kupffer cultures
represented by aryl sulphatase enzyme activity in normal and ALA recruited cells at
different pH and temperatures. Notice the low enzyme activity in normal cells while
maximum leucine degradation was at pH 6 and 40 °C.
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Enzyme Inhibition and Bioapplications
Fig. 22. Figure shows the effect of radiolabeled [14C]-Leucine in Kupffer cultures represented
by aryl sulphatase enzyme activity in normal and ALA recruited cells. Notice the threshold
of leucine turnover at 40 IU in terms of enzyme activity. Control cells showed slow %
protein degradation and low enzyme concentration. Source. Sharma 1990. Ph.D dissertation.
cultures added with trophozoits, nitroimidazole, exhibited increased protein degradation.
Lysosome as proteolytic component was shown to contribute for protein degradation as earlier
various reports indicated protein size dependence [Neff et al.1990]. Proteolytic rate in isolated
lysosomes increased several fold with decreased intralysosomal pH. However, leakage of
hydrolytic enzymes (elevated orotate phosphorylase/decarboxylase) may cause degradation
of extra lysosomal orotate substrates. Thus incubation time should be short. Degradation of
short and long-lived proteins in isolated lysosomes demonstrates that after 15 minutes of [14C]
leucine labeling, these lysosomes were active in degradation of short time labeled proteins.
Since transit time for lysosomal enzymes from endoplasmic reticulum to lysosomes was
around 2-3 hours [Zhu et al. 1994]. So after, 15 minutes some other proteins may be thought to
enter lysosomes. The degradation rate in isolated lysosomes was sometimes faster for short
time labeling because of short lived proteins as cytosolic or secretory in nature. On the other
hand, membrane proteins had half lives in range of few days. So these long labeled proteins
were degraded at different rates within lysosomes. Moreover short lived proteins were
reported more susceptible to digestion by lysosomal proteases. The effect of proteolytic
inhibitors on protein breakdown in isolated lysosomes for distinguishing lysosomal from extra
lysosomal protein degradation was dependent upon cell type, culture medium, labeling
schedule and inhibitors used. Inhibitors of lysosomal enzymes impede their supposed target
of action viz. the lysosomes. But complete inhibition of proteolysis could not be possible due to
lysosomal and nonlysosomal proteolysis. Pepstatin inhibited only selective aspartic proteinase
cathepsin D, leupeptin inhibited thiol proteinase. Similarly nitroimidazole either
intralysosomal pH and restored proteinases. Moreover, cathepsin D inhibition by pepstatin,
cathepsin B, H and L inhibition by leupeptin are reported commonly [Katunuma, 2011]. Thus
lysosomes were active in degradation of both short and long lived proteins during basal state.
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
235
Infected Kupffer cell lysosomes exhibited more pronounced elevated lysosomal degradation at
pH 5.0 at temperature range 30 C-40 C as shown in Figure 23.
Fig. 23. Figure shows a comparison of % proteolysis inhibition and % protein degradation in
presence of nitroimidazole, pepstatin and leupeptin. Proteolysis is represented in terms of
arylsulphatase enzyme activity. Notice the size of peptide length makes difference shown by *
for long duration and short duration of reaction in each additive. Control cells showed slow %
protein degradation and low enzyme concentration. Source. Sharma R 1990,Ph.D dissertation.
9. Liver regeneration and role of lysosomal enzymes
During metabolic integrity loss and resultant hypoxia state, other factors in liver cell signal the
alarm to produce nitric oxide, release of cytokines (IFN-α with IL-1β or TNF-α). These changes
also trigger the sequence of Kupffer cell stimulation and lysosomal action simultaneously. As a
result, lysosomal enzymes, growth factor, plasminogen stimulating factor consistently
participate or stimulate the Kupffer cell suicidal phagocytic action (hepatic necrosis) by
nitroimidazole analogs and previously reviewed [Michalopoulos, 1990; Thurman, 2000]. In
drug induced hepatitis, liver lysosomal enzymes have been reported elevated as stimulation of
hepatocellular defense apart from initiating respiratory burst and chemotaxis in liver cells
236
Enzyme Inhibition and Bioapplications
[Vician et al.2009]. Our study showed the associated electron microscopic observations that
Kupffer cells accumulate around the site of hypoxia and exhibited hyperplasia condition
showing degenerated nucleus, enlarged lysosomal vesicles [Sharma, 2009]. The enlarged
lysosomal vesicles were further correlated by elevated lysosomal enzyme levels in serum. In
liver biopsies, these lysosomal enzymes were significantly enhanced. Acid phosphatase,
leucine aminopeptidase enzymes catalyzing phosphorylation and amino-peptidization, seem
to be secreted more and suggestive of active protein degradation. β-glucuronidase and aryl
sulphatase high enzyme secretion was suggestive of continuous breakdown of aryl substituted
and β-glucuronidation reactions during cytolysis and proteolysis [Knook et al. 1981]. No
previous data is available on liver cell enzymes estimated in liver cells isolated from liver
biopsy of nitroimidazole treated livers in hypoxia. Our observations of enzyme inhibition in
hypoxic liver cells are very valuable to design a nitroimidazole based radiosensitizer or drug
for parasites, myocardial ischemia, tuberculosis, neurotoxicosis etc. Liver cell enzymes may
extend the better biochemical explanation of complex hypoxia state at molecular level.
However, several scattered studies on serum choline esterase, alkaline phosphatase, glucose6P-dehydrogenase, ornithine carbamoyl transferase, cyclooxygenase-2, lactate dehydrogenase
enzymes have been reported significant in hepatic hypoxia or hepatic damage evaluation with
addition of new members of enzymes [Carlson et al. 1976; Koppele et al. 1991]. Enzyme
inhibition in liver cells is significant information to design new liver cell metabolic drug
stimulators in detoxification, evaluation of hypoxia imaging probes, and cultivation of type of
monoaxenic liver cell cultures (hepatocytes and Kupffer cells) with specific enzyme profile.
There appear two main reasons of enzyme level hepatic cell recovery by nitroimidazole.
First, altered enzyme changes were recovered by nitroimidazole therapy as drug inhibits the
enzymes participating in energy status in hepatocytes and reduces Kupffer cell macrophagal
activity with reduced hepatic enzyme secretion to bring back enzyme levels to normal.
Second, regenerating hepatocytes may also regulate the normal cell recovery process and
initialize the signaling Kupffer cells to store enough lysosomal enzymes.
10. Role of enzyme inhibition in liver transplantation and tissue engineering
Liver regeneration after the enzyme inhibition or loss of hepatic tissue is a fundamental
parameter of liver response to injury. Enzyme inhibition phenomenon is recognized as
specific external stimuli involving sequential changes in gene expression, growth factor
production, and morphologic structure. Author sums up an account on different enzymes in
following description:

Angiotensin Converting enzyme inhibition by lisinopril degrades bradykinin and
subsequently induces liver regeneration in rats. During this enzyme inhibition process
(liver regeneration) induces 3.4 S DNA polymerase activity in both cytoplasmic fraction
and nuclear fractions as constant while 6 S to 8 S DNA polymerase activities in the
cytoplasmic fraction increased 6- to 7-fold [Ramalho et al.2001; Chang et al.1972]. The
activity of histidine decarboxylase was markedly increased in regenerating liver tissues
put. Ornithine decarboxylase is an enzyme catalyze in polyamine synthesis. Inhibition
of ornithine decarboxylase, histidine decarboxylase, and other amino acid
decarboxylases, was reported in regenerating rat liver and several rat tumors.[Russell,
et al.1969]. Amine synthesis in rapidly growing tissues: ornithine decarboxylase activity
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration


237
in regenerating rat liver and various tumors. Apart from enzyme inhibition, many
growth factors and cytokines such as most notably hepatocyte growth factor, epidermal
growth factor, transforming growth factor-α, interleukin-6, tumor necrosis factor-α,
insulin, and norepinephrine, appear to play important roles in this enzyme inhibition
and liver response. Many reviews attempted to integrate in last three decades and
looked toward clues to the nature of triggering mechanisms in liver and cellular
response. [Michalopoulos et al. 1997]. The major events are signaling cascades involving
growth factors, cytokines, matrix remodeling, and several feedbacks of stimulation and
inhibition of growth related signals. Liver manages to restore any lost mass and adjust
its size to that of the organism, while at the same time providing full support for body
homeostasis during the entire regenerative process [Michalopoulos et al. 2007].
Recently, role of tissue engineering concept showed several advantages in liver
transplantation. However, several factors are significant in regeneration: matrices
design, cell mass up to whole-organ equivalents, optimized microarchitecture, celladhesion peptides, growth factors and extracellular matrix molecules to the polymeric
scaffold, hepatic enzymes in microenvironment [Mooney et al. 1992]. The use of enzyme
characterization is very helpful in hepatocyte culture to test biocompatibility required
for the retention of tissue-specific gene expression [Kim et al. 2000; Voytik-Harbin et al.
1997]. Liver cell transplantation into polymeric matrices (cell-matrix-constructs) is
emerging as routine in hospitals to meet absolute requirement of the transplanted
hepatocytes for hepatotrophic factors that liver constantly receives through its portal
circulation [Starzl et al. 1973]. Apart from cellular therapies, two experimental
approaches are worth mentioning for successful clinical translation. The first “cell
sheet” technology developed by Sasagawa et al.,2007. Ott et al. reported perfusion
decellularization to generate whole organ scaffolds [Ott et al. 2008].
In previous report, investigators developed a similar perfusion decellularization method
for the liver [Baptista et al.2009] for organ bioengineering. These bioscaffolds preserve
their tissue microarchitecture and an intact vascular network that can be readily used as a
route for recellularization (intact enzymes of regulatory metabolic reactions) by perfusion
of culture medium containing different liver cell populations. Significant enzyme
inhibition assays are used as biomarkers of liver cell viability. In following example, a
bioengineered liver (the regenerated hepatic tissue) is shown clearly visible by the naked
eye after 7 days in the bioreactor (Fig. 24A). Regenerated tissue was highly cellular (Fig.
24B) and displays biliary duct structures positive for cytokeratin 19 (Fig. 24C), as well as
clusters of hepatocytes expressing cytochrome P450 3A and albumin (Fig. 24D, E,
respectively). Expression of endothelial cell nitric oxide synthase (eNOS) enzyme in
seeded human endothelial umbilical vein cells (hUVECs) were also observed coating and
spreading from vascular structures (Fig. 24F). In other approach, Uygun et al. 2010
decellularized rat livers and repopulated them with rat primary hepatocytes, showing
promising hepatic function and the ability of heterotopically transplanting these
bioengineered livers into animals for up to eight hours [Uygun et al. 2010]. Bioengineered
livers displayed some functions of a native human liver (albumin and urea secretion,
CYP450 enzyme expression, etc) and also exhibited an endothelial vascular network that
prevented platelet adhesion and aggregation, critical for blood vessel patency after
transplantation. Hence, this liver regeneration technology with proper active enzyme
functions has the potential to translate in the future into the bioengineering of human size
238
Enzyme Inhibition and Bioapplications
livers to offer readily available liver for drug discovery applications and for liver
transplantation, overcoming organ shortage.
Fig. 24. Human bioengineered livers are highly cellular and display some of the functions
observed in native hepatic tissue. (A) Macroscopic appearance of a seeded liver bioscaffold 7
days after seeding with primary human fetal liver and endothelial cells. (B) H&E showing
broad recellularization of the bioscaffold with the formation of biliary ductal structures. (C)
Immunofluorescence staining for cytokeratin 19 (red) showing a biliary duct formed within
the bioscaffold with a visible lumen. (D) Immunofluorescence staining of cytochrome P450
3A (orange) and (E) albumin (purple) showing groups of hepatocytic lineage cells
expressing these more mature hepatic markers. (F) Immunofluorescence staining of eNOS
(purple) showing hUVECs coating and spreading from vascular structures (arrows). All
nuclei stained with YO-PRO1 (green) or DAPI (blue). Reproduced with permission from
reference Baptista et al. 2010.
11. Challenges, limitations in enzyme inhibition
There are two major challenges. First challenge was to get enough biopsy to estimate several
enzymes. Second challenge was to choose significant enzymes as representative of
hepatocellular criteria. The main limitation was that the ‘hepatocellular hypoxia criteria’ was
used in small number of human subjects. The biopsy samples for electron microscopy
observations needs more thoroughly controlled experiments. Other limitation was that the
enzyme estimations in serum and biopsy samples may not be perfect representative samples
and it needs to establish the measurable and actual enzyme activities in cells. The most
crucial issue is that hypoxia is a combination of sequence of several metabolic and
subphysiological reactions in cells. Moreover, other inflammatory cytokines, apoptosis,
nitric oxide production and phagocytosis are associated changes during hypoxia. It further
needs tracer technique to track the details of hypoxia in cell.
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
239
12. Current developments and future prospects
Nitroimidazoles are rediscovered antiamoebic drugs having great potential of
radiosensitizers in diagnosis and treatment of tumors, tuberculosis, myocardial infarction,
and hypoxia. Nitroimidazole cytotoxicity was recently capitalized in hypoxic tumor
detection and tumor killing. Nitroimidazoles act as smart enzyme inhibitors on one hand
and play as enzyme modulators. Liver enzyme inhibition is unique for developing drugs
in drug discovery at molecular level. The major challenge is to explore enzyme inhibitor
that selectively and specifically catalyzes enzyme inhibition in only abnormal or diseased
liver cells leaving safe normal cells in acute stage of liver damage such as hepatic coma,
threatening hepatitis, hepatic cancer. The success of enzyme inhibition mainly depends on
intracellular physiological conditions such as pH, temperature, concentration of active
enzyme or inhibitor, coenzyme, co-factor, enzyme substrate structural-functional
conformation state, type of biochemical reaction such as coupled, cascade or individual
enzyme reaction and finally regulatory enzyme behavior to control metabolism.
Currently, art is developing in liver transplantation using cultured cells to replace
diseased or non-functional liver cells from diseased liver. Tissue engineering has opened
the new opportunity of using conditioned monoaxenic and monoclonal liver cells in both
experimental and pre-clinical or clinical therapeutics in both industries and academics.
Recently, gene therapy in liver was a great success and it will be a boom in medical
science of detoxification or drug metabolism if and when clinically accepted. Battery of
enzymes in hepatocytes and rate limiting enzyme regulatory behavior in presence of
additives or inhibitors will certainly enhance better understanding of disease
development and new drug pharmaceutical action in drug discovery. Still today, liver
cells remain as major research tool to discover drugs in therapeutics and search of
transplantable disease resistant liver cells because of liver as sole organ responsible of bile
formation, detoxification and drug metabolism. Cirrhosis is a serious health hazard that
solely requires functional liver. In future, it remains to see if molecular biology will assist
in silencing genes ON or OFF responsible of enzyme stimulation or inhibition in liver as
method of disease therapy and cure. The major discoveries will be discovery and better
understanding of new growth factors, colony stimulating factors affecting enzyme
reactions in hepatocytes and Kupffer cells as major engineered cancer resistant or
cirrhosis resistant liver cells. In future, chip technology or silica or polymer coated
enzyme estimation techniques may be more reliable in small sample collections or in liver
cells with high degree of accurate enzyme estimation.
13. Conclusion
The 2’nitroimidazole is both antiaparasitic drug and radiosensitizer hypoxia marker in
tumor therapy. It shows hepatocellular cytotoxicity. The ‘hepatocellular hypoxia criterion’
distinguishes the nitroimidazole induced hepatocellular oxygen depletion and associated
organelle changes by enzyme levels in serum, biopsy samples and cell organelles by
electron microscopy. Initially, glucose regulation leads to metabolic integrity loss. Later, it
may be oxygen insufficiency and slow cell death. The 2’-nitroimidazole is drug of choice
in hepatic infections and its derivatives are emerging choice of tumor treatment. Its action
may be evaluated rapidly by enzyme biochemical estimations without time consuming
drug monitoring and therapeutic assay techniques. Entamoeba histolytica, an intestinal
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Enzyme Inhibition and Bioapplications
protozoan parasite can proliferate, lyse and destroy human liver cells. Within the last
decade, new animal models of E. histolytica infection showed amoebic trophozoites cause
intestinal disease and liver abscess. Recent studies have expanded our understanding of a
large number of enzyme inhibition reactions in hepatic regeneration after liver abscess is
treated by nitroimidazole as remarkable drug to kill parasites. Still interactions between E.
histolytica and human intestine; and between E. histolytica and hepatocytes or Kupffer
cells are not fully known. In present chapter, a scheme of enzyme changes in liver cells
during development of amoebic liver abscess and enzyme inhibition is explored in detail
as rehabilitative action of nitroimidazole to regenerate liver cells. Characteristics of
enzyme regulatory behavior highlight the metabolic integrity loss, energy depletion,
oxidative phosphorylation, fatty deposition, phagocytosis and slow cell death as main
culprits in development of liver abscess and hypoxia. Nitroimidazole inhibits several rate
limiting enzymes to control oxygen (hypoxia) levels and keeps balance of NADPH, ATP,
fatty acids, amino acids and proteins. In hepatocytes, enzyme inhibition and regulatory
behavior of apoptosis (caspases, cysteine protease); carbohydrate metabolizing
(glucokinase, pyruvate kinase, phosphofructokinase); energy metabolism (NADPH
cytochrome P450 Reductase, Phosphodiesterase); DNA synthesis (Thymidylate
Synthetase) describe the process of liver regeneration. In Kupffer cells, enzyme inhibition
of drug metabolizing enzymes (oxidoreductases, proteases, esterases, phosphatases)
predicts the liver regeneration and recovery from liver abscess. In last decade, several
studies reported the use of enzyme inhibition mechanisms in enhancing liver regeneration
and recovery as tool of tissue engineering and liver transplantation. In conclusion,
enzyme inhibition mechanisms in liver damage and regeneration explain the better
information of liver transplantation to maintain complex liver cell environment in
balance. Enzyme inhibition mechanisms throw a light on characterization of enzyme
regulation in engineered liver cells, amoebic liver abscess development and desired
design of targeted nitroimidazole analogues to treat liver abscess.
14. Appendix 1: A protocol of hepatocellular dysfunction
Patients: In 10 control subjects, stool, ELISA, serum and liver biopsies were collected1. Only
proved 10 subjects free from any hepatic disease were examined for enzyme estimations in
isolated liver cells from biopsy specimens. Other ten subjects were treated with nitroimidazole
(Tiniba and Zil from Hindustan Lever Ltd, Bombay) therapy 2 x 5 gm one-time dose thrice at 2
days interval. At the end of dose, liver biopsy and serum collection was done.

All the 10 control subjects had no clinical findings of any hepatic disease and showed
stool -ve, ELISA -ve. All 10 showed normal liver and no abscess confirmed by
ultrasound and liver scan. The subjects were selected of 35 ± 15 years (mean age ± sd),
average monthly income 1750 ± 100 INR, no hepatomegaly, no diarrhea, fever and pain.
The subjects after nitroimidazole treatment showed induced hepatomegaly insignificant
less than 2 cm by liver scan. The subjects with amoebiasis and amoebic liver abscess are
not included and not shown here.
1The patients were studied for ongoing other research study on amoebic hepatitis vs amoebic liver
abscess by Hepatocellular Dysfunction Criteria at Liver Unit, AIIMS. The results of hepatic damage are
shown in Figure 3.
Mechanisms of Hepatocellular Dysfunction
and Regeneration: Enzyme Inhibition by Nitroimidazole and Human Liver Regeneration
241
The ELISA was done by method of Prakash et al.[44]. Liver biopsy was taken by Manghini
needle [45].
Biochemical assays: In biopsy sonipreps and serum samples the hepatocellular enzymes
were estimated as described elsewhere [Sharma, 2009]. All substrates of enzymes were
obtained from Sigma Chemical Company, St Louis, USA. The estimations were done on UV
spectrophotometer Cecil Inc. England [Sharma, 2009].
Electron microscopy: The biopsy specimens at stored in glutaraldehyde at -20°C. For fixing
samples were fixed in dental wax and emersed in pool of 0.1 M phosphate buffer containing
0.1 M Ca++. The samples were cut 1 mm cube by Gillette blade. The cubes were fixed for 2
hours in 0.2 M phosphate buffer in 4 changes at 30 minutes interval in capped vials. The
vials were washed for 2 hours in 0.2 M phosphate buffer 4 times at 30 minutes interval in
capped vials. The vials were post fixed in buffered 1% osmium tetroxide for 1-2 hours at
4°C, dehydrated in cold 10% ethanol for about 5 minutes followed by dehydrations in cold
50%, 70%, 80%, 95% ethanol for about 5 minutes each and tissues were kept at room
temperature. Further tissues were dehydrated at 100% ethanol for 15 minutes. Electron
Microscope PA model was used for screening hepatic cell damage by epoxy resin blocks and
osmium tetra oxide staining [46].
Art of liver cell isolation and enzyme characterization
The procedures of liver biopsy excision and preparation of liver cells were followed as per
ethical committee of institute as described in following section [Sharma, 2009].
Reagents Used
The collagenase enzyme was purchased from Sigma-Aldrich, St Louis, MO and pronase
enzyme was purchased from Boerhinger Meinheim GMbH.
Isolation and preparation of Kupffer cells: The liver biopsy samples (6-10 grams) were
digested with collagenase enzyme (Sigma-Aldrich, St Louis, MO) 15 mg/ml for 10 minutes
and bottom pallet was used for hepatocyte experiments. Other remained supernatant part of
digested liver was further digested with 2 mg/ml pronase enzyme (Boerhinger Meinheim
GMbH) in 0.2 % Kreb’s Hansleit Buffer pH 7.4 containing 5 mm glucose and 3 mM calcium
chloride after separation of hepatocytes. The said suspension was incubated for 45 minutes
at 37˚C stirring 250 revolutions per minute. After incubation, the cell suspension was filtered
through nylon sieve 79 × 79 microns and filtrate was centrifuged and washed twice with
KHB containing 5 mM glucose and 3 mM calcium chloride pH 7.4. After centrifugation and
final washing the filtrate was centrifuged at 350 × g in ice cold centrifuge. The settled pallet
at bottom was washed with TC 199 medium. The number of Kupffer cells was counted on
Naubourgh counting chamber and visibility was determined by trypan blue.
Kupffer cell fractionation: The isolated Kupffer cells were divided in two parts. One part
was used for Kupffer cellular enzymes and other part was used for in vitro experiments. For
enzyme estimations Kupffer cells from freshly harvested preparations were sonicated at
zero degree temperature in said KHB buffer in soniprep. Later these were fractionated for
different cellular fractions as described following by methods [Alabraba et al.2007; Heuff et
al. 1994; Kirn et al.1982].
242
1.
2.
3.
Enzyme Inhibition and Bioapplications
In refrigerated centrifuge above soniprep preparations were centrifuged at 1000 × g to
take out cell nuclear fraction and again centrifuged at 9000 × g to take out
mitochondrial fraction as pallet at bottom.
Above supernatant was centrifuged at 23000 × g to isolate microsomal bodies fraction
along with fraction rich in peroxisomes.
The light turbid supernatant was centrifuged at 100000 × g to isolate lysosomal fraction
as clear white pallet at bottom. Sometimes microsomes were contaminated with
lysosomes. The lysosomal purity evaluation was used to exclude microsomes free from
lysosomes [Wisse et al.1996].
Kupffer cells in cultures: The Kupffer cell cultures were maintained for 48 hours after
isolation from pronase perfused liver biopsy samples [Brouwer et al. 1988]. The in vitro
cultured cells were screened for trypan blue exclusion [Froh et al.2003]. The Kupffer cells
were used for their in vitro phagocytic action upon foreign particles which were sheep
erythrocytes, latex beads and zymosan suspended in 10 mM phosphate buffer saline pH 7.4
at 4°C. After harvesting Kupffer cells were added in phosphate buffered saline pH 7.4
containing 4.6 x 106 adherent cells and three ml PBS-TC 199 medium in culture flasks
containing Kupffer cells in mediukm with particle cell ratio 1:5 for erythrocytes, 1:40 for
latex beads and 1:4 for trophozoits. The ratio for zymosan and cell was 20:1. They were kept
up to 48 hours or more in carbogen atmosphere (95 % O2 and 5% CO2) in desiccator with
twice changes of medium after every 24 hours.
15. Acknowledgements
The author acknowledges the assistance provided by Professor Rakesh K Tandon to provide
assistant research officer job during this work with grant assistance from ICMR, New Delhi
and Dr V.S.Singh for guidance and valuable Council of Scientific and Industrial Research
fellowship and research grant from ICMR, New Delhi to do Ph.D.
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8
Reversible Inhibition of Tyrosine Protein
Phosphatases by Redox Reactions
Daniela Cosentino-Gomes and José Roberto Meyer-Fernandes
Universidade Federal do Rio de Janeiro (UFRJ),
Instituto de Bioquímica Médica (IbqM)
Brazil
1. Introduction
Protein phosphorylation/dephosphorylation is considered a widespread mechanism of
protein regulation. The covalent addition of a phosphate group to tyrosine residues in
cellular proteins may occur as an appropriate response to a series of morphological and
biochemical processes, with particular importance in complex functions such as growth,
proliferation, differentiation, adhesion and motility. Protein tyrosine phosphorylation is
regulated in the cell by the opposing activities of two classes of enzymes: protein tyrosine
kinases, which phosphorylate specific tyrosine residues in protein using ATP as the
phosphate source, and protein tyrosine phosphatases, which hydrolyze the
phosphotyrosines yielding the restored amino acid residue and inorganic phosphate as
products (Kolmodin & Åqvist, 2001; Mayer, 2008). The number of active protein
phosphatases with the ability to dephosphorylate phosphotyrosine are very similar to the
number of active tyrosine phosphatases. Both types of enzymes also display comparable
tissue distribution patterns (Alonso et al., 2004). Abnormalities in tyrosine phosphorylation
play a role in the pathogenesis of numerous inherited and acquired human diseases from
cancer to immune deficiencies (Alonso et al., 2004)
Phosphorylation, acetylation, ubiquitinylation and glycosylation are among the most wellknown post-translational modifications. The concept of protein oxidation as a posttranslational modification has only gained acceptance more recently. Protein oxidation
occurs as an outcome of a chemical attack by reactive oxygen species (ROS) or reactive
nitrogen species (RNS) on susceptible amino acids such as tyrosine, tryptophan, histidine,
lysine, methionine, and cysteine (Spickett et al., 2006). Indeed, it is important to note that
signal transduction must occur in a coordinated manner in response to a previous stimuli.
The key elements of a signaling response is reversibility and specificity (Tonks, 2005). In this
way, an oxidative-dependent chain reaction may be short and employ low concentration of
oxidants to avoid irreversible damage to cellular components.
In this chapter, we will discuss the following: the structural mechanism of action of ROS in
the enzymatic activity of tyrosine phosphatases and how it interacts with their target
molecules; the reversible regulation of this enzyme by oxidants and antioxidants; and the
major consequences of this tightly controlled mechanism on cell signaling.
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2. Protein tyrosine phosphatases family
The protein tyrosine phosphatase (PTP) superfamily is encoded by approximately 100 genes
in the human genome. They all possess an overall structure with a core catalytic domain
composed of four parallel β-strands surrounded on both sides by α-helices. The active site
sequence Cys(X)5Arg defines the PTP family, and this sequence is referred to as the “PTP
signature motif”. Moreover, this conserved motif creates a very similar structural motif,
termed the PTP loop, which connects a central β -strand to an α -helix at the center of the
catalytic site. The diversity within the families is conferred by the regulatory domains
(Salmeen & Barford, 2005; Tiganis & Bennett, 2007). All protein tyrosine phosphatases are
characterized by: a) their sensitivity to vanadate; b) an ability to hydrolyze the artificial
substrate p-nitrophenyl phosphate; c) an insensitivity to okadaic acid; and d) a lack of metal
ion dependence for catalysis (Chiarugi & Buricchi, 2007; Denu & Dixon, 1998). PTPs have
the capacity to function both positively and negatively in the regulation of signal
transduction, being able to stimulate or inhibit protein functions through their specific
dephosphorylation activity (Tonks, 2006).
The protein tyrosine phosphatase superfamily can be grouped into two subfamilies, the
tyrosine-specific phosphatases that comprise 38 different enzymes and the dual-specific
phosphatases with 61 components in the human genome. The common classification of the
protein tyrosine phosphatase family is depicted in table 1. Despite different subcellular
localizations and substrate specificities, the protein tyrosine phosphatase classes employ a
similar chemical mechanism for phosphate hydrolysis involving a transient cysteinylphosphate intermediate (Chiarugi et al., 2005; Östman et al., 2011).
2.1 Protein tyrosine-specific phosphatases
The subfamily of tyrosine-specific phosphatases is constituted by the well-known classical
protein tyrosine phosphatase (PTP), which acts strictly on phosphotyrosine-containing
proteins. The active site of this group is deeper than the catalytic site of dual-specific protein
phosphatase, which makes these enzymes much more selective for p-Tyr-containing
substrates than the dual-specific protein phosphatase. The classical PTPs can also be divided
into transmembrane, receptor-like enzymes (RPTPs) and the cytosolic, nonreceptor PTPs
(NRPTPs) (Chiarugi & Buricchi, 2007).
2.1.1 The receptor-like protein tyrosine phosphatases
The receptor-like PTPs such as RPTPα and CD45 are cell-surface receptors and generally
have an extracellular ligand-binding domain, a single transmembrane region, and one or
two cytoplasmic catalytic domains. Receptor-like PTPs are mainly negatively regulated
through dimerization, a known regulatory mechanism for many signal transduction
molecules, such as receptor proteins (Chiarugi & Buricchi, 2007). The number of PTP
catalytic domains encoded by the human genome is greater than the number of PTP genes
because many of the receptor-type proteins have tandem catalytic domains (Alonso et al.,
2004). Receptor-like tyrosine phosphatases have the potential to regulate signaling through
ligand-controlled protein tyrosine dephosphorylation. Many of these tyrosine phosphatases
display features of cell-adhesion molecules in their extracellular segment and can be
implicated in processes that involve cell-cell and cell-matrix contact (Tonks, 2006).
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
255
2.1.2 The intracellular protein tyrosine phosphatases
The intracellular protein tyrosine phosphatases contain a single catalytic domain and
various amino or carboxyl terminal extensions. They may contain SH2 (Src homology
domain) domains that have targeting or regulatory functions, PEST (Pro-Glu-Ser-Thr
domain) domains for proteolytic control, phosphorylation sites for protein/protein
interactions, and enzymatic activity regulation. Cytosolic tyrosine phosphatases are
characterized by regulatory sequences that flank the catalytic domain and control activity
either directly, by interactions at the active site that modulate activity or by controlling
substrate specificity. This class of enzymes is also activated through its own tyrosine
phosphorylation, being essentially inactive under normal basal conditions. Among the
members of this group are the prototype PTP1B, the most studied tyrosine phosphatase, the
low molecular weight-PTP (LMW-PTP), and the Shp2 phosphatase (Alonso et al., 2004;
Chiarugi & Buricchi, 2007).
Tyrosine phosphorylation is common to all cytosolic enzymes, including those that do not
contain an SH2 domain, such as PTP1B or LMW-PTP. Both phosphatases have been reported
to be activated through autophosphorylation. At the same time, autodephosphorylation has
been reported for several tyrosine phosphatases, including Shp2, RPTPα, Shp1, and LMW-PTP
(den Hertog et al., 1994; Giannoni et al., 2005; Meng et al., 2002; Zhao et al., 1994). PTP1B action
encompass numerous substrates, including different PTK receptors, such as the EGFR
(epidermal growth factor receptor), PDGFR (platelet-derived growth factor receptor), CSF-1
(colony-stimulating factor 1), IR (insulin receptor) and IGF-1 (insulin-like growth factor-1)
receptors, and cytoplasmic PTKs members, such as c-Src and JAK2 (Janus protein tyrosine
kinase 2). The non-transmembrane Shp1 and Shp2 (protein tyrosine phosphatase Src
homology 1/2 (SH2) containing domain) perform opposing tasks in signaling pathways. Shp1, predominantly expressed in hematopoietic cells, is a negative regulator in the signaling
pathways mediated by the chemokine and cytokine receptors and by other receptor tyrosine
kinases. Unlike other tyrosine phosphatases that negatively regulate signaling, Shp2
phosphatases can positively regulate cell signaling through the dephosphorylation of
substrates that are negatively regulated by tyrosine phosphorylation and can participate in the
propagation of the ERK (extracellular-signal-regulated kinase) and PI3K (phosphoinositide 3kinase)/Akt (protein kinase B) pathways in which multiple receptor PTKs play a role (C. Y.
Chen et al., 2009; Tiganis & Bennett, 2007).
2.2 The dual-specific tyrosine protein phosphatases
The dual-specific protein phosphatases (DSPs) can dephosphorylate a variety of substrates,
including phosphotyrosine-containing proteins, phosphothreonine, phosphoserine residues
and phospholipids, and is the most diverse group in terms of substrate specificity because of
this feature. The dual-specific protein phosphatases are much more diverse than tyrosinespecific phosphatases and can be divided into several subgroups. Eleven of the 61 dualspecific PTPs encoded by the human genome are specific for the mitogen-activated protein
kinases (MAPK); the other ones can be represented by the well-known cell cycle regulators
Cdc25 phosphatases and the tumor suppressor phosphatase (PTEN), which is also able to
dephosphorylate lipid substrates (Alonso et al., 2004; Chiarugi & Buricchi, 2007). The dualspecific tyrosine protein phosphatases also include the VH1-like enzymes, which are related
to the prototypic VH1 DSP, a 20-kDa protein that is a virulence factor of the vaccinia virus
(Tonks, 2005).
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The MAPK phosphatases are characterized by dual phosphothreonine and phosphotyrosine
specificity and the presence of a CH2 region and other MAPK-targeting motifs. They can
dephosphorylate tyrosine and threonine residues within the activation loop of MAPKs. The
different specificities towards the various MAPKs mean that MAPK phosphatase plays a
critical negative regulation on the MAPK-mediated signaling process (Tiganis & Bennett,
2007). Cdc25 phosphatases are involved in the dephosphorylation of cyclin-dependent
kinases at their inhibitory, dually phosphorylated N-terminal Thr-Tyr-motifs. The reaction is
required for activation of these kinases to drive progression of cell cycle (Alonso et al., 2004).
Another subgroup of dual-specific protein phosphatases, the atypical dual-specific protein
phosphatases, includes a number of poorly characterized enzymes that lack specific MAPKtargeting motifs and tend to be much smaller enzymes. This is the case for PIR (phosphatase
interacting with RNA/RNP), which dephosphorylates mRNA. Cdc14 is involved in the
inactivation of cyclin-dependent kinases and in exit from mitosis, while the slingshots and
PRLs (phosphatase of regenerating liver) are poorly understood. PTEN dephosphorylates
phosphatidylinositol-3,4,5-trisphosphate at the plasma membrane, while the myotubularins
primarily dephosphorylate phosphatidylinositol-3-phosphate on internal surface of cell
membranes (Alonso et al., 2004).
Regarding substrate specificity, dual-specific protein phosphatases may present a preference
for either tyrosine or serine/threonine residues. For cyclin-dependent kinase (CDK)associated phosphatase (KAP), the phosphatase preferentially dephosphorylates the
threonine residue in the activation loop of CDKs, while for the VH1-related dual-specific
phosphatases, tyrosine is the preferred residue in MAPKs (Tonks, 2006).
Table 1. Classification of the protein tyrosine phosphatase family and some of its principal
members and ligand-inducing oxidation, according to Tonks in 2006. (PDGF, plateletderived growth factor; EGF, epidermal growth factor; TNFα, tumor-necrosis factor-α;
TCPTP, T-cell protein tyrosine phosphatase; MKP, mitogen-activated protein kinase
phosphatase; LMW-PTP, low molecular weight protein tyrosine phosphatase).
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
257
3. Structural characteristics of the cysteine-based protein tyrosine
phosphatase catalytic domain
Protein tyrosine phosphatases are a family of enzymes with high structural multiplicity.
Cysteine-based phosphatases share the common function of hydrolyzing phosphorester
bonds in proteins and/or lipids via a conserved cysteine-based mechanism. The most
significant feature of the protein tyrosine phosphatase superfamily is the presence of a
signature motif Cys(X)5Arg, where X is any amino acid. This well-conserved motif forms the
phosphate-binding loop in the active site, known as the P-loop or PTP-loop, and is
responsible for structural features required for phosphate recognition and phosphorester
hydrolysis. The structural arrangement ensures that the catalytic cysteine functions as the
nucleophile in catalysis, which specifically binds negatively charged substrates, and the
arginine, which is involved in phosphate binding, remains in close proximity to form a
cradle to hold the phosphate group of the substrate in the correct place for nucleophilic
attack (Chiarugi & Buricchi, 2007; Salmeen & Barford, 2005; Tabernero et al., 2008). Figure 1
shows the crystal structure of PTP1B complexed with phosphotyrosine.
Fig. 1. (A) Crystal structure of PTP1B complexed with phosphotyrosine (PDB entry 1PTV)
(Jia et al., 1995). A substrate positioned in the active site is represented by phosphotyrosine
(ball and stick). The cysteine (C215) represented in the active site is the nucleophile that
attacks the substrate phosphorous atom, forming the cysteinyl-phosphate intermediate. The
arginine (R221) is involved in the intermediate stabilization and substrate binding
(Tabernero et al., 2008). (B) Binding of phosphotyrosine to the catalytic site. Here the
nucleophile cysteine 215 is mutated to serine 215 (Jia et al., 1995).
Specific protein tyrosine phosphatases utilize a two-step reaction for phosphate monoester
hydrolysis (Fig. 2). The first step is initiated by a nucleophilic attack by the active-site
cysteine (C215) on the phosphorus atom of the bound substrate that leads to the formation
of a cysteinyl-phosphate intermediate. Besides substrate binding, the arginine is also
involved in the stabilization of this intermediate. During the formation of the transition-state
intermediate, the only aspartic residue in the WPD (Trp-Pro-Asp) loop acts as a general acid
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Enzyme Inhibition and Bioapplications
by donating its proton to the phenolic oxygen of the tyrosyl-leaving group, thus cleaving the
phosphate of tyrosine to form the cysteine-phosphate intermediate. This first substitution
reaction allows the phosphate group to be covalently attached to the nucleophile via a
thioester linkage. In the second step of catalysis, the same aspartic residue functions as a
general base by accepting a proton from water during hydrolysis of the cysteinyl phosphate
intermediate, thus restoring the free enzyme to its normal basal Cys-SH conformation
(Kolmodin & Åqvist 2001; Tabernero et al., 2008; Tiganis & Bennett, 2007). Figure 2
illustrates the general two-step catalysis of protein tyrosine phosphatases.
Most of the cysteine residues within proteins present a normal pKa of 8.3, and this
characteristic ensures that this amino acid has a relatively good nucleophile attribute.
Because of the unique environment of the tyrosine phosphatase active site, the catalytic
cysteine presents an unusually low pKa value. Unlike the free cysteines, which are shown to
be protonated at physiological pH, the catalytic cysteines of the tyrosine-specific PTPs are
more acidic and are therefore proposed to be deprotonated in the free enzymes at a
physiological pH (or even at the pH optimum of 5-6), existing as a thiolate anion (Cys-S−).
This property enables them to act as nucleophiles in the first step of catalysis but also makes
them highly susceptible to oxidations by different reactive oxygen species and, to a lesser
degree, with reactive nitrogen species. Exposure of thiolate anions to reactive nitrogen
species causes S-nitrosothiol formation, whereas treatment with peroxynitrite yields Snitrothiol formation (Brandes et al., 2009; Chiarugi et al., 2005; Salmeen & Barford, 2005).
Fig. 2. Schematic mechanism of two-step catalysis of protein tyrosine phosphatases. The first
step is initiated by a nucleophilic attack of the active site cysteine, which is in its thiolate
anion conformation (Cys-S−), on the phosphorus atom of the bound substrate (1). The
dephosphorylated protein tyrosine is released from the active site of the phosphatase while
a cysteinyl-phosphate intermediate is formed. In the second step of catalysis, the aspartic
residue in the P-loop accepts a proton from water during hydrolysis of the cysteinyl
phosphate intermediate, thus restoring the free enzyme to its basal Cys-S− conformation (2).
After oxidation, the modified cysteine can no longer function as a phosphate acceptor in the
first step of catalysis, which abrogates the catalytic function of tyrosine phosphatase
(Chiarugi et al., 2005; Salmeen & Barford, 2005). A network of hydrogen bonds stabilizes the
negative charges in the active site. Among other interactions in the PTPs, it is found that the
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
259
hydroxyl group of the serine or threonine residue in the signature motif is important for
stabilizing the thiolate form of the active site cysteine, which results in the characteristically
lower pKa. The P-loop is found at the N-terminus of an α-helix and should also contribute
to the thiolate stabilization, as well as, the presence of neighbouring acid (Asp or Glu) and
basic (Arg, His or Lys) amino acids near the catalytic cysteine (Hess et al., 2005; Kolmodin
and Åqvist, 2001).
The structure of cytoplasmatic tyrosine phosphatases is highly conserved with only minor
differences in the main structural core. The catalytic domain is formed by about 280 amino
acids, which is responsible for the specific folding of the enzyme and other differing
characteristics. The catalytic domain often exhibits relatively broad substrate specificities in
vitro; however, under in vivo conditions, full-length tyrosine phosphatases have more
stringent specificities. This is in part due to the presence of additional regulatory domains
that direct their subcellular localization and interactions with specific substrates (Tabernero
et al., 2008).
4. Sources of oxidizing species in biological systems
4.1 Reactive oxygen species
Reactive oxygen species (ROS) are a group of molecules produced in cells when oxygen is
metabolized and are more reactive than molecular oxygen (Groeger et al., 2009). In aerobic
organisms, ROS can originate from different sources, including mitochondrial electron
transport chain, xanthine oxidase, myeloperoxidase, nicotinamide adenine dinucleotide
phosphate (NADPH) oxidases (Nox enzymes), and lipoxygenase (Groeger et al., 2009;
Östman et al., 2011; Salmeen & Barford, 2005). The last two are responsible for the
production of ROS in response to hormones, growth factors and cytokines (Bae et al., 1997;
Salmeen & Barford, 2005; Sundaresan et al., 1995). Among these oxygen metabolites are
superoxide anions (O2•-), hydrogen peroxide (H2O2) and hydroxyl radicals (•OH) (Chiarugi
& Buricchi, 2007). These species are not all equally reactive regarding to prospective targets.
Many of them have very short half-lives, leading to little relevance in terms of signaling. For
example, the hydroxyl radical is the most unstable radical, reflecting its limited ability to
transmit signals across any significant distance. At the same time, superoxide and hydrogen
peroxide can be considered more stable species and because of these, may be the most
favorable ROS to operate as a signaling molecule (K. Chen et al., 2009).
After being considered important components of the biological process by McCord and
Fridovich in 1969, free radicals emerged in the subsequent years as a dangerous byproduct
of cellular metabolism by bringing deleterious consequences to DNA, lipids and proteins.
The first research on reactive oxygen species has focused primarily on their chemical ability
to react with and cause toxic and mutagenic effects on cellular components. There are
exceptions, such as phagocytes, that deliberately generate reactive oxygen species as a main
defense mechanism (Lambeth, 2002). Almost ten years later in 1977, Mittal and Murad
showed the first evidence of the potential for free radicals to act in cellular signaling in
metabolism. Now, it is largely accepted that living organisms have not only adapted to cope
with oxidizing species but also have developed mechanisms for the advantageous use of
free radicals (Dröge, 2002). ROS can act as second messengers that are required for the
downstream signaling events (Cross & Templeton, 2006) However, mechanisms by which
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Enzyme Inhibition and Bioapplications
proteins are able to sense ROS and translate signaling into an effective response are not
totally clear.
Superoxide anions are derived by the univalent reduction of triplet-state molecular oxygen
(3O2). This process is mediated by enzymes, such as xanthine oxidase, NADPH oxidases
(Nox) and dual oxidases (Duox) (also known as Nox enzymes), or nonenzymatically by
redox reactive compounds, such as the semi-ubiquinone of mitochondria. Within
mitochondria, superoxide is continually generated as a result of electron leakage from the
electron transport chain mainly at complex I (NADH/ubiquinone oxidoreductase) and
complex III (ubiquinol/cytochrome c oxido-reductase) (Cross & Templeton, 2006; Harrison,
2002).
Nox Members
Expressing cells
Main characteristics
Nox 1
Colon, vascular smooth muscle, prostate
Nox 3
Fetal liver, fetal lung, fetal spleen, fetal
kidney
Nox 4
Kidney, osteoclasts, pancreas, placenta,
skeletal muscle, ovary, astrocytes
Nox 5
Spleen, sperm, lymphnode, ovary,
placenta, pancreas
Presents a calmodulin-like domain
and can be activated by calcium.
Duox 1
Thyroid, lung
Duox 2
Thyroid, colon
Presents a calmodulin-like
domain, and an amino-terminal
peroxidase domain.
Similar to phagocytic Nox 2
because of their structure and
enzymatic activity.
Table 2. Nox members in nonphagocytic cells.
Xanthine oxidase catalyzes the oxidation of xanthine to uric acid and O2•-. Further O2•- can
be dismuted into hydrogen peroxide and molecular oxygen. The enzyme is expressed
mainly in the liver and epithelia (Harrison, 2002).
Regarding the Nox family, seven mammalian members are known. The seven members are
Nox1-5 and dual oxidases (Duoxs) 1 and 2. Nox proteins are integral membrane proteins
with six transmembrane domains, which together are thought to form a channel to allow the
successive transfer of electrons. They operate by transferring electrons from NADPH
(converting it to NADP−) to FAD, then to heme and finally to oxygen to make O2•-. Duox1
and 2 both have a peroxidase-like domain in their N-terminal region, which means that
these two members of the family produce hydrogen peroxide and not O2•- (Groeger et al.,
2009). The NADPH oxidase complexes were firstly thought to be specific to macrophages,
but now these enzymes are recognized and expressed almost universally in nonphagocytic
cell types, and are also able to produce ROS via a similar mechanism (Chiarugi & Buricchi,
2007; Cross & Templeton, 2006). The wide-ranging distribution of Nox members, suggests
that ROS production may have a broad implications for the different cellular phenotypes (K.
Chen et al., 2009). Besides the NADPH oxidase from phagocytic cells (Nox 2), Nox family can
be divided into three groups that present distinct regulatory patterns from those of Nox 2. The
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
261
three groups of nonphagocytic Nox members are summarized in table 2. The production of
reactive species by NADPH oxidases occurs during inflammatory conditions in cells of the
immune system or by external stimuli, such as growth factors in most mammalian cells, and
their relevance is related to intracellular signaling (Dorgë, 2002; Lambeth, 2002).
Lipoxygenase is a mixed function oxidase involved in the synthesis of leukotrienes from
arachidonic acids. Which are released from membrane phospholipids via the activity of
cytosolic phospholipase A2, and are also responsible for the production of superoxide
(Chiarugi & Buricchi, 2007; Kim et al., 2008).
Superoxide is not highly reactive and cannot cross cellular membranes; its diffusion is
dependent of anion channels in the membranes. In most cases, the potential effects of
superoxides are restricted to a single intracellular compartment that can be directed by some
organelles like mitochondria, endosomes, and phagosomes (K. Chen et al., 2009). However,
superoxide can readily be converted to hydrogen peroxide by superoxide dismutase, which
finally penetrates membranes (Cross & Templeton, 2006).
Hydrogen peroxide is a two-electron oxidant that acts as an electrophile and can react with
protein thiol moieties to produce a variety of sulfur oxidation states, as will be discussed
later. Most of the times, this reactivity affords reversible posttranslational modification of
proteins that would be important for cellular signaling. Moreover, the relatively longer halflife of hydrogen peroxide compared with other ROS in biologic systems affords better
activity as second messengers (K. Chen et al., 2009). Both, superoxide and hydrogen
peroxide are involved in the inhibitory oxidation of the catalytic sites of the protein tyrosine
phosphatases.
4.2 Reactive nitrogen species
The reactive nitrogen species (•NO) is produced in higher organisms by the oxidation of one of
the terminal guanidino-nitrogen atoms of L-arginine. This process is catalyzed by the enzyme
nitric oxide synthase (NOS). Nitric oxide (NO) can be generated by the reduction of NOx
species that are derived from exogenous or endogenous sources (Hess et al., 2005). Depending
on the microenvironment, NO can be converted to various other reactive nitrogen species
(RNS), such as nitrosonium cation (NO+), nitroxyl anion (NO−) or peroxynitrite
(ONOO−)(Dröge, 2002). Separate genes code for different NOS isoforms, for example, neuronal
NOS (nNOS or NOS1) and for endothelial NOS (eNOS or NOS3), which are named after the
tissues in which they were discovered, as well as, for inducible/Ca2+-independent NOS (iNOS
or NOS2). (Hess et al., 2005). Nitric oxide is produced in large amounts, ranging in nanomoles
concentration, for several hours. In contrast, the constitutive NOS found in some tissues
remains active for relatively short periods of time (Do et al., 1996). The attachment of the NO
moiety to a nucleophilic group or a transition metal is called nitrosyl. Ingeneral, the NO
moiety can be provided by NO itself, nitrite, other NOx species (higher NO oxides), metal-NO
complexes. Nitric oxide has high reactivity for thiol groups that are present in proteins,
reduced glutathione, or cysteine residues. The chemical addition of NO moiety to a reactive
cysteine thiol group (nitrosation) is called S-nitrosylation (SNO). S-Nitrosylation-induced
conformation can lead to the formation of S-nitrosoproteins, S-nitrosoglutathione, or Snitrosocysteine. These nitrosylated molecules can also serve as source of NO moieties. (Do et
al., 1996; Janssen-Heininger et al., 2008). Accumulation of protein SNOs is the only mechanism
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Enzyme Inhibition and Bioapplications
of NO toxicity that is able to induce nitrosative stress. Nitrosative stress that results from
increased or deregulated production of reactive nitrogen species is countered by several
cellular mechanisms triggered by nitrosative modifications of proteins that mobilize adaptive
and protective responses (Hess et al., 2005).
4.3 Antioxidant mechanisms
4.3.1 Antioxidants of reactive oxygen species
The transitory fluctuations in ROS provide important regulatory functions, but when
present in high amounts, they can induce severe damage to cellular components. To avoid
further deleterious outcomes, cells have evolved with the ability to handle increases in ROS
production. These include various nonenzymatic and low-molecular-weight antioxidants,
for example, reduced glutathione, vitamins A, C, and E, flavonoids, and enzymatic
scavengers (Chiarugi & Buricchi, 2007). Reduced glutathione is a tripeptide synthesized in
cells by the sequential addition of cysteine to glutamate followed by the addition of glycine,
giving rise to the molecular γ-L-glutamyl-L-cysteinyl-glycine. The sulfhydryl group (-SH) of
the cysteine can be involved in reduction reaction, removing peroxides, or it can be
associated in conjugation reactions with enzymes such as glutathione peroxidases (GPx) and
the isoforme 6 of peroxiredoxins (Psdx 6). These enzymes catalyze the reduction of
hydrogen peroxide by GSH into H2O and oxidized glutathione (GSSG). In mammalian cells,
GSH is present in millimolar concentrations and the conversion of GSH to its oxidized form
GSSG by enzymatic activities serves as a redox buffer system within cells. The GSSG can
then be reduced back originating two GSH molecules by the action of glutathione reductase
enzymes, at the expense of NADH or NADPH (Cross & Templeton, 2006; Forman et al.,
2009). Glutathione also is important for maintaining ascorbate (vitamin c), itself a potent
intracellular antioxidant, in a reduced state (K. Chen et al., 2009).
In cells, superoxide can be converted nonenzymatically to hydrogen peroxide and singlet
oxygen (1O2), or through the enzymatic action of superoxide dismutases (SOD), superoxide
can be then converted into H2O2, which is a less reactive specie. In mammals, three isoforms
of superoxide dismutase are recognized: the cytoplasmatic SOD, which is a copper/zincdependent dismutase (SOD1); the mitochondrial manganese SOD (SOD 2); and the
extracellular Cu/Zn-dependent SOD (SOD3).
In the presence of reduced transition metals, such as ferrous or cuprous ions, hydrogen
peroxide can be converted into a highly reactive hydroxyl radical (HO−). This radical is a
less stable radical species and acts in a nonselective, oxidative manner on organic molecules.
There are several families of enzymes, e.g., catalases, glutathione peroxidases (GPx), and
peroxiredoxins (Prx), that break down hydrogen peroxide directly (Dröge, 2002; Salmeen &
Barford, 2005). Catalases are localized entirely in peroxisomes and efficiently converts
hydrogen peroxide to water and molecular oxygen, while glutathione peroxidases are
largely restricted to the cytosol and mitochondria. Glutathione peroxidases reduce
peroxides by transferring electrons from GSH with the generation of oxidized glutathione.
Peroxiredoxins exhibit a higher affinity toward hydrogen peroxide and are also abundant in
the cytosol. Peroxiredoxins remove hydrogen peroxide to yield water, while form an
intermolecular disulfide bond, which then can be reduced by thioredoxins (Trx).
Consequently, thioredoxins are not directly involved in the removal of ROS, but indirectly
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
263
through a supportive role to the peroxiredoxins. Besides their function as ROS scavengers,
these enzymes can actually protect proteins from oxidations or regenerate oxidized proteins
(K. Chen et al., 2009; Chiarugi & Buricchi, 2007). The overall catalysis mechanism of these
antioxidant enzymes is summarized below:
SOD
 H2O 2 + O 2
O 2•  O 2•  2H+ 
(1)
H2 O 2 + Fe 2+  Fe 3+ + • OH + OH-
(2)
Catalase
2H2 O 2  2H 2 O + O 2
GPx or Prx
(3)
4.3.2. Antioxidants of Nitrosylated Molecules
As mentioned above nitrosative stress results from increased or deregulated production of
reactive nitrogen species (Hess et al., 2005). The formation of S-nitrosoglutathione is
regulated by the coordinately action of the enzyme S-nitrosoglutathione reductase (GSNO
reductase), also known as glutathione-dependent formaldehyde dehydrogenase. This
enzyme converts GSNO to GSNHOH, ultimately forming ammonia and oxidized
glutathione. GSNO reductase is responsible for the regulation of the steady-state levels of Snitrosylated proteins, that are in equilibrium with GSH. Denitrosylation can also be
achieved by specific metal ions, such as Cu+, the low-molecular-weight antioxidant
ascorbate, or thioredoxins enzymes (Janssen-Heininger et al., 2008).
5. Redox inhibition of cysteine-based protein tyrosine phosphatases
Reversible oxidation of regulatory proteins by hydrogen peroxide or NO/SNO has been
implicated in broad cellular signaling mechanisms. Tyrosine phosphatases can be
differentially regulated by phosphorylation, intra- and intermolecular interactions,
alternative splicing, transcription, and translation, but recently, it has become apparent that
this family of enzymes may also be regulated by reversible oxidation. In general, the redox
regulation of proteins occurs primarily when cysteine residues within the protein or bound
to transition metals react with ROS, such as H2O2 or superoxide, or with reactive nitrogen
species, such as nitric oxide, S-nitrosothiols, or peroxynitrite. Reactions of cysteines with
mild oxidizing or nitrosylating reagents, such as low concentrations of H2O2 or •NO, can
lead to reversible modulations of protein activity. Oxidation or nitrosylation of the catalytic
cysteine of protein tyrosine phosphatases renders the residue unable to act as a nucleophile,
and the enzyme loses its phosphatase activity (Chiarugi et al., 2005). Protein phosphatases
are among the earliest characterized targets of inhibition by thiol group modification. Thiol
group modifications are thought to play a central role in cellular signaling process through
ROS modifications (Cross & Templeton, 2006).
The first evidence of protein regulation by the redox modulation of a cysteine was described
in 1967 by Pontremoli and colleagues, which showed that disulfide formation with
cysteamine stimulated fructose-1,6-bisphosphatase activity. The redox regulation of tyrosine
phosphatases depends on different oxidative stress conditions and is tissue specific
(Chiarugi et al., 2005).
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Enzyme Inhibition and Bioapplications
Dual
oxidase
NADPH
oxidase
Lipoxygenases
•−
O2
Xanthine
oxidase
SOD
H2O2
Cell
signaling
Fig. 3. Sources of reactive oxygen species. Superoxide (O2•-) can be produced from different
sources as within the mitochondrial electron transport chain, the activity of xanthine
oxidases, Nox members like NADPH oxidase or Dual oxidase, and lipoxygenases. Later,
superoxide may be dismuted to hydrogen peroxide (H2O2) through the action of superoxide
dismutase (SOD). H2O2 can then act as second messenger in cell signaling.
For tyrosine phosphatases, the typical element of redox regulation occurs within the
catalytic site of the enzyme. Two specific amino acid residues are essential to the catalysis of
tyrosine phosphatases, Cys215 and Arg221. Oxidation of the catalytic cysteine of the protein
tyrosine phosphatases occurs in vivo in response to intracellular ROS production, which can
occur in response to a variety of stimuli, such as platelet-derivative growth factor (PDGF),
epidermal growth factor (EGF), insulin, several other cytokines, and integrins (Rhee et al.,
2003; Tonks, 2005). Cysteine residues are also affected by extracellularly added ROS or by an
imbalance of the redox potential within the cell. Therefore, hydrogen peroxide, Snitrosylation, or other radical species may oxidize cysteine residues in proteins to form
cysteine sulfenic acid (Cys-SOH), which can be further stabilized by the formation of interor intramolecular disulfide (S-S) or sulfenyl-amide bonds. The last conformation is
characterized by a five-membered ring that is formed by the covalent linkage of an atom of
the catalytic cysteine to an amide nitrogen in a neighboring residue (Chiarugi & Buricchi,
2007; Denu & Tanner, 1998; Janssen-Heininger et al., 2008).
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
265
Different cysteine residues can take part in the disulfide bridge, such as what may occur
with the catalytic cysteine of LMW-PTP or PTEN with another cysteine residue or between
the classical protein tyrosine phosphatases such as PTP1B and a cysteine from GSH (Barrett
et al., 1999; Chiarugi et al., 2001; den Hertog et al., 2005; Kwon et al., 2004; Lee et al., 1998;
Salmeen & Barford, 2005). The tripeptide GSH can also induce alterations of proteins
function by the formation of mixed disulfide bonds, a reversible process referred to as
glutathionylation. The last conformation can result in a temporarily inhibition of PTP
activity that protects enzyme from irreversible oxidation. Glutathionylation can be easily
reversed by the action of glutaredoxins or thioredoxins, restoring normal enzyme function
(Cross & Templeton, 2006).
It is assumed that both disulfide bonds or sulfenyl-amide conformations protect the reactive
cysteine from a higher-order, irreversible oxidation or expose the oxidized cysteine to
cellular reducing agents, which facilitates the reactivation of the oxidized cysteine, because
sulfenyl-amide rings or disulfide bonds represent reversibly oxidized states of cysteine and
can be readily reduced (Chiarugi et al., 2005; Salmeen & Barford, 2005; Tonks, 2005;
Tabernero et al., 2008). Moreover, these two conformational changes may represent a signal
that the given protein tyrosine phosphatase is in a temporarily inactive state (Tabernero et
al., 2008), which may be important to stabilize associations within protein complexes, to
modify structures, to create, destroy or modulate functional sites, or to regulate enzymatic
or transcriptional activities of proteins (Poole & Nelson, 2008).
Dual-specific phosphatases, such as Cdc25, PTEN, and LMW-PTP, and classical protein
tyrosine phosphatases are also a target of ROS oxidation. Nevertheless, these enzymes
contain a second cysteine residue within the active site. Following oxidation of the cysteine
in the signature motif, a disulfide bond is formed with the vicinal cysteine, which protects
the enzyme from higher oxidation and irreversible inactivation. Intramolecular disulfide
bond formation involving the catalytic cysteine of classical PTPs has not been observed (den
Hertog et al., 2005; Lee et al., 2002; Tonks, 2005).
Different phosphatases present distinct sensitivities to oxidation. Regulation of the oxidized
states of the cysteine may vary with the structural distances between the cysteines involved
in the formation of disulfide bridge and the availability of thiolate ions. This may represent
a degree of specificity for protein tyrosine phosphatase redox regulation and lead to a
different extent of oxidation (Chiarugi & Buricchi, 2007).
Inactivation of the protein tyrosine phosphatases by hydrogen peroxide could be reversed
by thiol-reducing agents such as dithiothreitol (DTT), β-mercaptoethanol, reduced
glutathione, cysteine or by the action of some enzymes in which their catalytic cycle
involves either a mono- or dithiol mechanism, such as glutaredoxins, thioredoxins, and
protein disulfide-isomerase that have a CXXC motif in the active site. Thioredoxins are able
to reduce sulfenic acids and disulfide bonds, while glutaredoxins are involved in the
reversibility of mixed disulfides with GSH and S-nitrosothiols (Chiarugi & Buricchi, 2007).
The oxidative modification of protein tyrosine phosphatases can be stable over prolonged
periods (Salmeen and Barford, 2005). In some cases, the sulfhydryl group is open to further
irreversible oxidation if no cysteine derivatives or compounds containing thiol groups are
close enough to facilitate the formation of a disulfide bridge or a sulfenyl-amide ring. The
addition of one or two oxygen molecules results in the formation of sulfinic acid (Cys-SO2)
266
Enzyme Inhibition and Bioapplications
or sulfonic acid (Cys-SO3), respectively. These oxidative modifications are irreversible, and
the phosphatase will be unable to become active again, even in a reducing environment
(Groeger et al., 2009). However, it has been shown recently that sulfinic acids can be reduced
by the action of a specific enzyme called sulfiredoxin, which has only been described in
mammals and yeast so far (Biteau et al., 2003; Woo et al., 2003).
The redox balance of the cells can affect the ability of ROS to oxidize a given protein. At
normal cellular conditions, the cellular redox potential may vary from –260 mV to –150 mV. In
addition, protein oxidation will depend on the distance between the site of ROS production
and the target protein and on the concentration of scavenger enzymes available. The length of
time that the protein will be inactivated will also depend on the proximity to enzymes such as
glutaredoxin and thioredoxin, which catalyze thiol reduction (Salmeen & Barford, 2005).
Regarding protein tyrosine phosphatases, the differences in pKa values of catalytic cysteines,
the availability of thiolate ions and the structural distance between the two cysteines involved
in the disulfide bridge may also strongly determine the ability of the phosphatases to be
rapidly regulated by changes in the intracellular redox conditions, thereby representing a
degree of specificity for tyrosine phosphatase regulation (Chiarugi et al., 2005).
Reversible
Reduction
PTP-S-N
Irreversible
Sulfenylamide
H2O
Amino
acid-NH
ROS
PTP-SThiolate
anion
Reduction
ROS
PTP-S-OH
Sulfenic acid
Sphix
ROS
PTP-S-O2H
Sulfinic acid
PTP-S-O3H
Sulfonic acid
Protein-SH
H2O
Reduction
PTP-S-S-Protein
Disulfide with protein
Fig. 4. The redox modulation of a protein tyrosine phosphatase thiolate anion. Because of its
low pKa value, the cysteine residue in the active site of the tyrosine phosphatase is present in
its thiolate anion form. This conformation facilitates the inhibition of the enzyme by oxidation,
which would become inactivated as the sulfenic acid form. Furthermore, sulfenic acid is
converted to a more stable conformation, such as a disulfide bridge or sulfenyl-amide ring.
Both conformations are stable and reversible through the action of reductant agents. However,
if oxidation persists for a long period, stronger and higher oxidation could lead to the
formation of sulfinic and sulfonic acids, which are irreversible conformations. In some cases,
sulfinic acid can be reversed enzymatically through the action of sulfiredoxin (Sphix) enzyme.
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
267
6. Cellular signaling through oxidative inhibition of the protein tyrosine
phosphatases
6.1 Receptor tyrosine kinases
Cellular signaling processes are controlled by the coordinated activity of protein
phosphatases and kinases. Protein tyrosine kinases (PTKs) are usually divided into two
families, the transmembrane receptor family and the nonreceptor family (Blume-Jensen &
Hunter, 2001). Receptor tyrosine kinases (RTKs) are cell-surface, transmembrane receptors
carrying a multidomain extracellular segment that binds polypeptide ligands, a single-pass
transmembrane helix, and a cytoplasmic segment containing a tyrosine kinase domain
together with several regulatory sequences located both N- and C-terminal to the kinase
domain. The receptor tyrosine kinase family includes epidermal growth factor receptor
(PDGFR), fibroblast growth factor receptor (FGFR), and the insulin receptor kinase (IRK),
among others (Chiarugi & Buricchi, 2007).
The kinase activities of protein tyrosine kinases are closely regulated through
autoregulatory mechanisms and by the action of tyrosine phosphatases. In general, most
tyrosine kinases are maintained in a low activity state through a variety of these
autoregulatory mechanisms and by avoiding the most favorable configuration of the kinase
active site. Activation of receptor tyrosine kinases is achieved through a ligand binding to
the extracellular domain, which stabilizes a dimeric receptor arrangement and facilitates
trans-phosphorylation in the cytoplasmic domain. Autophosphorylation of one or two
conserved tyrosines within the juxtamembrane domain of these receptors is required for
complete kinase activation (Chiarugi & Buricchi, 2007).
6.2 Hydrogen peroxide as a second messenger in cellular signaling
The classic receptor-mediated signaling involves a ligand engagement, followed by the
production of a diffusible second messenger that interacts with a target to affect the signal.
This arrangement supports both signal transduction over space and signal amplification,
because most second messengers are produced via an enzymatic process. Typically, second
messengers are small, diffusible molecules that rapidly activate effector proteins such as
protein kinases, phosphatases or ion channels. These molecules can act through binding or
chemical modification, thus promoting signal transduction (K. Chen et al., 2009).
It is now widely accepted that hydrogen peroxide may function as a second messenger
within cells by being able to induce tyrosine phosphorylation of cellular proteins, which is
strongly potentiated by a combination treatment with vanadate. The mechanism was
believed to be attributed to the inhibition of tyrosine phosphatase, the activation of tyrosine
kinases, or both (Chiarugi & Buricchi, 2007). Furthermore, the addition of exogenous
hydrogen peroxide to cells can mimic the effects of growth factors or hormones and lead to
hyperphosphorylation of receptor tyrosine kinases. Mitogen-activated protein kinases have
also been shown to be activated by extracellular hydrogen peroxide through a specific
kinase cascade (Torres, 2003).
Little is known about how hydrogen peroxide is actually distributed to the cytoplasm. Most
of the time, Nox family members are responsible for producing hydrogen peroxide near the
receptor at the plasma membrane; in addition, hydrogen peroxide, which is produced in
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Enzyme Inhibition and Bioapplications
response to stimulation by a ligand, is released outside of the cell to then be diffused locally
into the cell. Also, NADPH oxidase and lipoxygenase, which can be localized within the
endoplasmic reticulum and the nuclear membrane, are able to release hydrogen peroxide
into the luminal spaces of the specific organelles (Chiarugi & Buricchi, 2007; Tonks, 2005).
Until recently, it was thought that hydrogen peroxide would cross biological membranes
freely; however, many membranes were shown to be almost impermeable to hydrogen
peroxide as well as to water (Antunes & Cadenas, 2000; Makino et al., 2004; Seaver & Imlay,
2001). Thus, for transport, hydrogen peroxide must be carried by an appropriate transporter
that has not yet been well characterized (Chiarugi & Buricchi, 2007).
6.3 Nitric oxide as a second messenger in cellular signaling
Nitric oxide is a gas, which readily diffuses across membranes and does not act at
conventional membrane associated receptors (Do et al., 1996). Like hydrogen peroxide,
signal transduction through induced-nitric-oxide modifications relies on the system of Cysbased posttranslational modifications. Accordingly, S-nitrosylation of proteins plays an
essential role in downstream cascades.
Nitric oxide exerts a ubiquitous influence on cellular signaling in large part by means of Snitrosylation/denitrosylation of protein cysteine residues. SNO, or higher oxides is
considered a post-translational protein modification that is precisely regulated, conferring
specificity to NO-derived effects. These NO-dependent modifications influence protein
activity, protein-protein interactions, and protein location. S-nitrosylation thus serves as the
prototypical redox-based signal (Janssen-Heininger et al., 2008).
S-Nitrosylation has been implicated in transmitting signals downstream of all classes of
receptors, including G-protein-coupled receptor (GPCR), receptor tyrosine kinase, tumor
necrosis factor, Toll-receptors, and glutaminergic receptors, acting locally within subcellular
signaling domains as well conveying signals from the cell surface to intracellular
compartments, including the mitochondria and the nucleus (Janssen-Heininger et al., 2008).
Regarding cellular signaling through S-nitrosylation, specific features of this kind of
modification can be considered useful tools in signaling transduction According to JanssenHeininger et al. (2008). These features include:
1.
2.
3.
4.
5.
Temporal regulation of response through a rapid and controlled stimulation;
The existence of motifs within proteins that provides S-nitrosylation specificity;
Colocalization of target proteins with a source of NO;
Reversibility of protein S-nitrosylation;
Enzymatic control of S-nitrosylation through the action of S-nitrosoglutathione
reductase.
6.4 The role of protein tyrosine phosphatases in cellular signaling
Physiological stimuli, such as growth factors or the engagement of antigen receptors, can
trigger localized and controlled production of ROS. Protein tyrosine phosphatases, because
of their high susceptibility to inactivation by oxidation at physiological pH, are targets of
ligand-induced ROS generation. Thus, the ability of tyrosine phosphatases to
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
269
dephosphorylate receptor tyrosine kinases becomes temporarily compromised because of
the oxidation of the catalytic site of the tyrosine phosphatases. These events are involved in
the enhancement of the signaling response through an overall increase in tyrosine
phosphorylation. Therefore, complete activation of the tyrosine kinase signaling cascades
requires both an inhibition of protein tyrosine phosphatases by oxidation and an activation
of receptor tyrosine kinases by phosphorylation (Salmeen & Barford, 2005). Redox signaling
through the reversible oxidation of tyrosine protein phosphatases must occur through a
reaction that is chemically reversible under physiological conditions to ensure the continuity
of cellular mechanisms.
Fig. 5. After ligand engagement, the receptor tyrosine kinase (RTK) needs the first phase
in which the tyrosine phosphorylation level must be high to guarantee signal
propagation. This high level is granted by a transient inhibition of protein tyrosine
phosphatase (PTP) due to oxidation promoted by hydrogen peroxide and by the
disrupted association with the receptor substrates. Hydrogen peroxide is produced by
NADPH oxidase that also became activated by ligand stimulation of the receptor tyrosine
kinase. The concomitant tyrosine phosphorylation of oxidized protein tyrosine
phosphatase, which is performed in response to receptor tyrosine kinase activation, is
ineffective on the abolished enzymatic activity and preparative for the second phase. In
the signaling termination phase, the tyrosine phosphatase recovers its enzymatic activity
due to a re-reduction of the catalytic thiol group and immediately becomes hyperactive
due to the previous tyrosine phosphorylation, thus promoting the rapid and efficient
termination of the signal. The tyrosine phosphatase recurrence after oxidation is followed
by a dephosphorylation of the activated receptors, which consequently terminates the
signal (Chiarugi et al., 2005).
Besides their regulatory functions, the regulatory domains in tyrosine phosphatases are also
involved in the sensitivity to oxidation, which probably contributes to the intrinsic
sensitivity differences between various protein tyrosine phosphatases to oxidative inhibition
270
Enzyme Inhibition and Bioapplications
(Groeger et al., 2009; Östman et al., 2011). The redox regulation of tyrosine phosphatases
may depend on different oxidative stress conditions and the extent of oxidation, as it is
tissue and differentiation dependent (Chiarugi et al., 2005). As mentioned above, the
transient negative regulation of tyrosine phosphatases due to oxidants is dependent on
ligand stimulation of the receptor tyrosine kinases.
Hydrogen peroxide is recognized as one of the main ROS capable of acting as a second
messenger because of it is relative stability. Cytokines, growth factors and integrins
activate the multicomponent NADPH oxidase enzymes as part of the receptor-mediated
signaling in cells. Both superoxide and, hence, hydrogen peroxide are generated, with the
latter serving a particularly significant role as a second messenger in signaling, which
mediates the regulation of survival pathways in cells (Poole & Nelson, 2008; Groeger et
al., 2009).
The tyrosine phosphorylation level of a given protein is the result of the collective action
of kinases and phosphatases. Signal transduction by ROS through reversible tyrosine
phosphatase inhibition represents a widespread and conserved mechanism that is
triggered by the ligand stimulation of receptor tyrosine kinases. Conformational changes
lead to an increase in ROS production, which causes a transient negative regulation of
protein tyrosine phosphatases that are associated with receptor tyrosine phosphatases,
and at the same time decreases receptor tyrosine phosphatase dephosphorylation that
leads to a high stimulation of its activity and signal propagation. The same PTP that
suffered oxidation can be highly phosphorylated by the protein tyrosine kinase that is
now activated. The oxidation of the active site of the tyrosine phosphatase protects the
enzyme from autodephosphorylation. When the tyrosine phosphatase returns to the
reduced form, its highly phosphorylated state influences a hyperactivation of the
enzyme’s activity, which is essential to terminating the signaling cascade in a fast way
(Chiarugi & Buricchi, 2007). This mechanism represents a strategy adopted by cells to
promote receptor tyrosine kinase signaling by avoiding its prompt inactivation by
tyrosine phosphatases. After the redox potential has been restabilized, tyrosine
phosphatase recovers its activity and the signaling triggered by the receptor tyrosine
kinases is terminated (Chiarugi et al., 2005; Chiarugi & Buricchi, 2007). Thus, tyrosine
kinase activation by oxidation is considered an indirect effect of tyrosine phosphatase
inhibition. The overall mechanism is represented in figure 5.
The distinct but complementary function of kinases and phosphatases clarifies the role of
both enzymes in controlling cellular signaling. Therefore, it is possible to affirm that kinases
are implicated in controlling the amplitude of a signaling response whereas phosphatases
are thought to have an important role in controlling the rate and duration of the response
(Tonks, 2006).
A key element of a signaling response is reversibility. Regulated reversible
phosphorylation of proteins and other cellular molecules plays a ubiquitous role in the
control of cellular behavior. For oxidation to illustrate the mechanism for reversible
regulation of protein tyrosine phosphatase function, it is essential that the active site
cysteine residue not be oxidized to sulfinic or sulfonic acids, which is usually irreversible
(Poole et al., 2004; Tonks, 2005). As mentioned elsewhere, thiol groups (SH) do not react at
Reversible Inhibition of Tyrosine Protein Phosphatases by Redox Reactions
271
physiologically significant rates with a hydroperoxide, such as hydrogen peroxide, unless
the reaction is catalyzed. However, thiolates (S−) react faster with hydroperoxides
depending on their local environment (Forman et al., 2004). The reaction rate of hydrogen
peroxide with protein tyrosine phosphatase is about 105 times slower than with
peroxiredoxin. The question appears on how hydrogen peroxide can react specifically
with the active site of PTPs before its metabolization by enzymatic scavengers. Two
possible mechanisms can arise:
1.
2.
Hydrogen peroxide is generated by NADPH oxidase close to PTP so that the
concentration could be sufficient to cause enzyme modification; or
PTPs could be compartmentalized to the sites of ROS generation and far from scavenger
enzymes (Chiarugi et al., 2005; Forman et al., 2004).
Besides, specificity of ROS to a given PTP may be influenced by contributions from the
surrounding amino acids near the catalytic cysteine. The prevalence of basic amino acids
near the active site of the enzyme can contribute to the extension formation of thiol
ionization giving rise to thiolate anions within active cysteine. Chemical reactivity
secondary to the local amino acids environments is likely to contribute to the modification
of cysteine residues (Cross & Templeton, 2006). This is the case of disulfide bonds or
sulfenyl-amide conformations.
Receptor signaling in which receptor activation induces both the activation of
downstream kinases and the oxidative inactivation of inhibitory phosphatases are
components required for efficient signaling. The regulated and localized production of
ROS can inactivate a subpopulation of phosphatase molecules near the site of ROS
generation. The specificity of ROS for tyrosine phosphatase exists through different
intrinsic sensitivities to oxidation and a tight control over the production of ROS. This
production can be achieved by the controlled action of Nox enzymes, including their
requirement for assembly into a multiprotein complex, the phosphorylation of regulatory
subunits, the binding of inositol phospholipids and the small GTPase, Rac. In addition,
regulation of Duox and NADPH oxidases by Ca2+ adds further control of ROS generation
in cells (Tonks, 2006; Poole et al., 2004). As an example, Nox4 can regulate the oxidation of
PTP1B in response to insulin and localizes with PTP1B on the intracellular membranes.
Ecto-phosphatases, which are protein phosphatases that present a catalytic site facing the
extracellular medium, are also able to respond to the endogenous production of hydrogen
peroxide from mitochondria. The addition of a proton ionophore, which was able to
reduce mitochondrial hydrogen peroxide production, was also shown to induce ectophosphatase activity due to the reduction of hydrogen peroxide outside of the cell
(Cosentino-Gomes et al., 2009).
7. Conclusion
The mechanism by which ROS/RNS exerts a regulatory function in cysteine-based protein
tyrosine phosphatases is well known, as well as the role of antioxidants in reverse enzyme
oxidation. The reversible inhibition of tyrosine phosphatases by ROS has been intensely
investigated in some diseases such as cancer and diabetes, but the complete pathway in
which these reactive species are involved is not elucidated yet. Some questions still
272
Enzyme Inhibition and Bioapplications
remain unsolved such as: How exactly ROS/RNS has a specific action on a given tyrosine
phosphatase? Is there a compartmentalization of proteins involved in redox pathways? Is
there a specific site of ROS/RNS production to induce redox signaling? How much of the
total hydrogen peroxide generated in a specific site can be translated into a cellular
signaling event? Which molecules would be involved in the redox sensor of the cell? The
challenge to solve these questions seems to be very difficult, since we have multiple
oxidants species from several sites of the cell. The development of specific probes for a
given reactive specie, or for a particular site of ROS/RNS production would be of great
contribution to understand the role of redox reactions in the regulation of physiological
signaling process.
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9
Feasible Novozym 435-Catalyzed Process to
Fatty Acid Methyl Ester Production from Waste
Frying Oil: Role of Lipase Inhibition
Laura Azócar, Gustavo Ciudad, Robinson Muñoz,
David Jeison, Claudio Toro and Rodrigo Navia
Scientific and Technological Bioresources Nucleous, La Frontera University
Chile
1. Introduction
Fatty acid methyl ester (FAME) or biodiesel is a biofuel conventionally produced from
edible oil and methanol, using an alkaline catalyst, through a transesterification reaction.
As FAME is mostly produced from edible vegetable oils, crop soils are used for its
production, increasing deforestation and producing a fuel more expensive than diesel. In
addition, between 70 and 80% of the total FAME production costs correspond to the
vegetable oils. Therefore, the use of waste lipids such as waste frying oils (WFO), waste
fats and soapstock has been proposed as low-cost alternative to feedstock. Non-edible oils
such as jatropha, pongamia and rubber seed oil are also economically attractive. In
addition, microalgae, bacteria, yeast and fungi with 20% or higher lipid content are
oleaginous microorganisms known as single cell oil and have been proposed as feedstock
for FAME production. Alternative feedstocks are characterized by their elevated acid
value due to the high level of free fatty acid (FFA) content, causing undesirable
saponification reactions when an alkaline catalyst is used in the transesterification
reaction. The production of soap consumes the conventional catalyst, diminishing FAME
production yield and simultaneously preventing the effective separation of the produced
FAME from the glycerin phase. These problems could be solved using biological catalysts,
such as lipases or whole cell catalysts, avoiding soap production since the FFAs are
esterified to FAME. In addition, by-product glycerol can be easily recovered and the
purification of FAME is simplified using biological catalysts.
Lipase-catalyzed processes have been widely investigated for FAME production from
alternatives raw material. Although interesting results have been reached up to date, the
enzymatic catalysis has not become competitive compared to the conventional chemical
process. The main reasons explaining this issue are the long reaction time (until 48 h), the
loss of enzymatic activity due to methanol use in the reaction and the high operational costs
because the lipases cannot be reused. The present chapter described an investigation to a get
a feasible lipase-catalyzed process to FAME production from WFO, avoiding lipase
inhibition.
278
Enzyme Inhibition and Bioapplications
2. Lipase catalyzed process to FAME production from waste frying oil:
Improving the yield
In spite of that some investigations have been carried using WFO instead of edible oils in
FAME production using lipases, there is not clear the effect in the process of replace the raw
material. Using Rhizopus oryzae as the biocatalyst, FFA from a synthetic WFO were esterified
to produce FAME with an improved reaction yield (Li et al., 2007). In addition, using
Thermomyces lanuginosus lipases immobilized on a microporous polymer, 97% FAME
content from edible sunflower oil was reached, while only 90.2% FAME content was
obtained from WFO (Dizge et al., 2009). Watanabe et al. (2001) tested the immobilized the
Candida antartica lipase immobilized on acrylic resin (Novozym 435) and obtained a 5.5%
reduction in FAME conversion yield when using WFO compared to edible oil as the
feedstock. They concluded that the oxidized fatty acid compounds in WFO may be
responsible for this decrease.
As the effects of using WFO instead of edible oil in lipase-catalyzed processes are not clear,
the aim of this study was to elucidate the effect of WFO incorporation in feedstock mixed
with rapeseed oil on FAME production yield using Novozym 435 as the catalyst by means
of the response surface methodology (RSM). In addition, specific WFO and rapeseed oil
chemical characteristics were investigated to identify the components that were responsible
for these results. Finally, a preliminary study to establish the optimal time for methanol
addition during the reaction was proposed.
Both filtered WFO collected from restaurants and crude rapeseed oil from a local factory
from Southern Chile were used as the feedstocks. Novozym 435 from Sigma-Aldrich was
used as the catalyst. Methyl heptadecanoate, 1,2,3-butanetriol and 1,2,3-tricaprinoylglycerol
were used as internal standards and were chromatographically pure.
RSM was used to analyze and optimize the interaction effects of four variables on FAME
production yield (Table 1): the WFO content in the feedstock mixture (% wt), the final
methanol-to-oil ratio (mol/mol), temperature (°C) and Novozym 435 dosage (% wt based on
oil weight). A central composite matrix with 5 levels was used and 30 runs were carried out
in a random order. Each run was performed in triplicate.
All reactions were incubated in flasks containing 1 mL of oil at 200 rpm. The volume of the
flasks was selected to maintain perfect agitation of the samples when using both the highest
dosage of catalyst and the lowest methanol-to-oil molar ratio. Under these conditions,
different combinations of feedstock mixture, dosage of catalyst, methanol-to-oil molar ratio
and temperature were used. Methanol was added in two steps to avoid lipase inhibition
(Shimada et al., 2002). In the first step, one-third of the total molar ratio was added
according to Table 1, while in the second step, the remaining two-thirds of the total molar
ratio was added to generate the final methanol-to-oil molar ratio.
To establish the reaction and second methanol addition times, a preliminary study was
performed. This experiment was carried out for 48 h using the RSM central point, with 50%
(wt) WFO in a mixed feedstock, a methanol/oil molar ratio of 3:1 and 9% (wt) Novozym 435
at 45°C and stirring 200 rpm.
Samples were immediately stored at 4°C to stop the reaction. The upper layer was analyzed
by gas chromatography for FAME quantification.
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
Independent variables
279
Levels
Symbols
-2
-1
0
1
2
WFO in feedstock [wt %]
X1
0
25
50
75
100
Methanol to oil ratio [mol/mol]
X2
1.50
2.25
3.00
3.75
4.50
Temperature [°C]
X3
35
40
45
50
55
Novozym 435 [wt %]
X4
3
6
9
12
15
Table 1. Variable and levels used in the response surface methodology
The experimental data obtained were fitted to a second-order polynomial equation. This
equation describes the relationship between the predicted response variable (FAME
production yield) and the independent variables (Table 1). The polynomial model for FAME
yield may be written as follows (Eq. 1):
4
4
i=1
i=1
FAME yield=β 0 + β i X i + βii X i2 +
4
 βijXi X j
(1)
i<j=1
Where β (0 = intercept, i = linear, ii = quadratic and ij = interaction) and Xi, Xj (i = 1, 4; j = 1,
4; i ≠ j represent the coded independent variables) are the model coefficients. With the fitted
quadratic polynomial equation, contour plots were developed to analyze the interaction
between terms and their effects on FAME production yield.
To identify and quantify the fatty acids in the feedstock a Clarus 600 chromatograph
coupled to a Clarus 500T mass spectrometer of Perkin Elmer (GC-MS) was utilized. An
Elite-5ms capillary column with a length of 30 m, thickness of 0.1 µm and internal diameter
of 0.25 mm was used. The vials were prepared by adding 3 µg of sample to 100 µL methyl
heptadecanoate as an internal standard (initial concentration of 1300 mg/L). The following
temperature program was used: 50°C for 1 min and then increasing temperature at a rate of
1.1°C/min up to 187°C. Both the injector and detector temperatures were 250°C and He was
used as the carrier gas. Before injection into the GC-MS equipment, the WFO and rapeseed
oil were methylated according to Araújo (1995). Specific gravity was measured at 20°C using
a manual densimeter. Kinematic viscosity was measured at 40°C using a capillary
viscosimeter. The acid value was determined by titration with KOH using phenolphthalein
as an indicator. The peroxide value was determined by titration with Na2S2O3, and the
iodine value was determined by the Wijs method (Araújo 1995). The following parameters
were measured according to ASTM Standard Methods: water and sediments (ASTM
Standard D 1976-97), pour point (ASTM Standard D 97-04), sulfur (ASTM Standard D 703904) and flash point (ASTM Standard D 56-02a).
To establish the degree of oil conversion, monoacylglycerols (MG), diacylglycerols (DG),
triacylglycerides (TG) and FFA in the oil feedstock were quantified using an HP 6890 series
gas chromatography system (GC-MS) with an adaptation of the EN-14214
(The)methodology. TG was determined by mass balance. A 50-m long BPX-5 column with a
thickness of 0.5 µm and an internal diameter of 0.32 mm was used. The vials were prepared
by combining 10 mg of sample with 0.8 µL of 1,2,3-butanetriol and 10 µL of 1,2,3tricaprinoylglycerol dissolved in pyridine as internal standards. N-methyl-N-
280
Enzyme Inhibition and Bioapplications
trimethylsilyltrifluoroacetamide (MSTFA) was added to the vials, which were then shaken
and incubated for 15 min to transform the sample into more volatile siliade components.
Subsequently, the preparation was dissolved in 0.8 mL of n-heptane. The following
temperature program was used: 15ºC for 1 min and three consecutive ramps of 15ºC/min to
180ºC, 7ºC/min to 230ºC and 10ºC/min to 320ºC and 320ºC for 15 min. The detector
temperature was 380ºC and a split rate 10 was used for injection of 1 µL of sample.
Methanol addition. Methanol solubility is less than 1.5:1 methanol/oil (mol/mol), but a molar
ratio of 3:1 methanol/oil is necessary to complete the transesterification reaction for FAME
production (Shimada et al., 2002). However, very high initial concentrations of methanol
could lead to the inhibition of the lipase used in this study due to its low solubility in oil (Du
et al., 2004; Shimada et al., 2002). Therefore, the experiment was designed so that the
methanol was added in two steps (Fig. 1).
100
1.0
productivity
[g FAME/g catalyst/h]
FAME content [%]
80
60
40
20
0.8
0.6
0.4
0.2
0.0
0
10
20
30
time [h]
30
40
0
0
10
20
40
50
50
60
time [h]
Fig. 1. FAME content and productivity during the reaction time. Operational conditions: 50
wt% WFO in the mixed feedstock, methanol to oil molar ratio of 3:1, 9 wt % Novozym 435
and 45°C, at 200 rpm. Arrows indicate methanol addition.
The experiment was started with an initial concentration of one-third of the necessary moles
of methanol. When about one-third of the oil was converted to FAME after 8 hours, the
remaining two-thirds of the methanol were added (Fig. 1). The addition of the higher
concentration of methanol in the second step was determined based on Shimada et al.
(2002), who established that methanol is more soluble in FAME than in TG.
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
281
This two-step technique of methanol addition was shown to diminish the reaction time by
obviously preventing enzyme inhibition. Li et al. (2009) showed that in spite of the high
yield reached using stepwise methanol addition, both the reaction time and rate of FAME
conversion were slower. In experiments using a three-step methanol addition and 4% (wt)
Novozym 435 as the catalyst, high FAME production yields were reached but only after 50
hours of reaction (Du et al., 2004; Watanabe et al., 2001). Similarly, in another recent study,
an reaction time of 50 hours was also used (Ognjanovic et al., 2009). Usually, the second
methanol addition starts after one-third (33.3%) of the FAME content has been generated
and the initially added methanol has been completely consumed (Du et al., 2004; Shimada et
al., 2002). This methodology produces a stationary phase of FAME production yield,
diminishing the reaction productivity.
As Novozym 435 was shown to be a robust and stable catalyst compared to other lipases in
the presence of short chain alcohols (Hernandez-Martin et al., 2008), all of the experiments
in this study were performed using this commercially available immobilized enzyme. When
approximately 28.6% of the FAME content was reached after 8 hours of reaction, the second
aliquot of methanol was added. This stepwise addition of methanol increased the reaction
productivity by avoiding the stationary production phase and led to a plateau in FAME
production at 12 hours. Similar reductions in the reaction time by shortening the stationary
phase of FAME production have already been reported (Li et al., 2009). Therefore, in the
RSM experiments, the first amount of methanol (one-third of the molar ratio shown in Table
1) was added at the start of the reaction, while the second aliquot of methanol (two-thirds of
the molar ratio shown in Table 1) was added after 8 hours, and the total reaction time was
12 hours.
Optimization of methanolysis conditions. Four variables previously shown to have significant
effects on FAME production yield were investigated. The methanol-to-oil ratio, temperature
and Novozym 435 dosage were established based on previous studies in the literature (Du et
al., 2004; Fukuda et al., 2001; Kose et al., 2002). According to previous experiments (data not
shown), the effect of the WFO content in the feedstock mixture was also tested in the lipasecatalyzed process. The experimental central composite design matrix is presented in Table 1.
To understand and optimize the relationship between the tested variables, the obtained
experimental data were analyzed by second-order polynomial equations by means of the
RSM. The analysis of variance (ANOVA) of the quadratic polynomial model showed low pvalues (0.125) and both high determination coefficients (R2=95.3%) and high adjustment of the
determination coefficients (Adj. R2=94.5%). The low p-value obtained indicates that the model
accurately represented the relationship between response and the variables. The R2 value
obtained indicates that the variation in FAME production yield correlated with 95.3% of the
independent variables and the obtained Adj. R2 value indicates a 94.5% correlation between
the independent variables. The lack of fit refers to the fact that a simple linear regression model
may not adequately fit the experimental data. The obtained p-value not indicate a significant
lack of fit and, therefore, gave no reason to evaluate a more complex model.
The p-values obtained from the regression analysis results showed that all of the coefficients
of the linear, interaction and quadratic terms had a significant effect on FAME production
yield. All of the linear and quadratic terms, as well as the interaction terms X1X3, X2X4 and
X3X4, were significant at the 1% level. The interaction terms X1X2, X1X4 and X2X3 were
282
Enzyme Inhibition and Bioapplications
significant at the 5% level. Therefore, the final response model equation in terms of the
variable factors can be written as follows (Eq. 2):
FAMEyield  896.52  1.90  X1  91.40  X 2  28.06  X 3  20.00  X 4  0.01  X12
 14.46  X 22  0.24  X 32  0.85  X 42  0.12  X1  X 2  0.02  X1  X 3
(2)
 0.03  X1  X 4  0.59  X 2  X 3  4.13  X 2  X 4  0.26  X 3  X 4
The main factors that affect the FAME production yield were the linear terms X1, X2, X3 and
X4, the quadratic term X22 and the interaction term X2X4. The linear terms with positive
coefficients indicate an increase in FAME production yield. As X1 is a positive term, it is
possible to establish that WFO incorporation could increase FAME production yield.
To validate the model obtained, an experimental point was compared to the model
prediction (Eq. 2), and an error of less than 8% was obtained. In addition, the optimal
methanolysis conditions were obtained through the regression model (Eq. 2) according to
the limit criterion of the maximum response of FAME production yield. The obtained
optimal conditions were 100% (wt) WFO, a methanol-to-oil ratio of 3.8:1 (mol/mol), 15%
(wt) Novozym 435 and 44.5°C, which generated 100% FAME production yield. These
predicted conditions show that WFO incorporation increases FAME production yield.
Moreover, using this model, it was also possible to predict several optimal conditions to
reach the highest FAME production yields, while simultaneously reducing production costs
and enzyme use. For instance, 96% FAME production yield could be reached using 75% (wt)
WFO in the mixed feedstock, with a methanol-to-oil ratio of 3.75:1 (mol/mol), 12% (wt)
Novozym 435 and 40°C.
To illustrate that several optimal combinations are able to produce the highest FAME
production yields, contour plots were generated. Figure 2 and 3 show the effect of variable
interaction on FAME production yield in contour plots predicted by the model. The contour
plots were generated to show the effect of two variables on the response, while the other
two independent variables were held constant.
Figure 2 shows the interaction between two independent variables (WFO in the mixed
feedstock and Novozym 435 dosage) and their effects on the response variable FAME
production yield, while the other two variables were held at zero. At these conditions, an
increase in Novozym 435 dosage led to an enhancement in FAME production yield. This
change is additionally increased when WFO is incorporated at up to 80% in the mixed
feedstock. WFO seems to be a more available substrate compared to rapeseed oil for
Novozym 435 catalysis under the conditions investigated.
FAME production yield was more sensitive to Novozym 435 dosage, since high doses were
necessary to obtain high FAME production yields. In this sense, in order to decrease the
production costs for future economically sound industrial applications, further experiments
should be conducted to investigate the effect of WFO incorporation using another
inexpensive biologic catalyst instead of Novozym 435. In addition, because Novozym 435
has shown high residual activity in successive applications (Hernandez-Martin et al., 2008),
further experiments with a subsequent recovery protocol and reuse of the immobilized
catalyst should be conducted under the optimal conditions established in this work.
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
100
80.0
40.0
WFO in mixe d feedstock [%]
283
80
60
40
20
20.0
0
60.0
5.0
7.5
10.0
12.5
15.0
Novoz ym 435 [wt%]
Fig. 2. Contour plot of FAME production yield predicted from the model at 45ºC and molar
methanol to oil ratio 3:1 mol/mol.
Recently, Issariyakul et al. (2008) investigated the use of mixed WFO and rapeseed oil using
an alkaline catalyst. However, lower ester production yields were observed when WFO was
incorporated in the reaction mixtures. These results agreed with Fukuda et al. (2001), who
established that the FFA in WFO can be completely converted to FAME using lipases as
catalysts, whereas soap is produced when an alkaline catalyst is used, which diminishes the
production yield. Although the cost of lipases is significantly higher than that of alkaline
catalysts, their use may partially solve the main drawbacks of the conventional biodiesel
production process: the use of pure vegetable oils (which account for around 80% of the cost
of the total process) and the competition with food products (Gui et al., 2008). We have
shown that the incorporation of WFO in the feedstock to partially replace rapeseed oil in
processes catalyzed by Novozym 435 may diminish the production costs while
simultaneously increasing the production yield, which makes it a potential alternative
method for FAME production on an industrial scale.
FAME production yield was more sensitive to both the methanol-to-oil ratio and Novozym
435 dosage, compared to temperature (Fig. 3). FAME production yield was enhanced when
the temperature increased to 45-50ºC; however, the opposite tendency was observed for
temperatures higher than 50ºC. These results agree with the results obtained by Kose et al.
(2002), who established that at about 50ºC, FAME production yield decreases as a result of
enzyme deactivation at high temperatures (Fig. 3A). An enhancement in FAME production
yield was observed when both the methanol-to-oil ratio and Novozym 435 dosage increased
(Fig. 3B). When using a high immobilized biocatalyst load, large amounts of alcohol were
284
Enzyme Inhibition and Bioapplications
55
80.0
Temperature [°C]
40.0
80.0
50
45
40
20.0
35
5.0
60.0
7.5
10.0
12.5
Novozym 435 [wt%]
A
15.0
Methanol to oil molar ratio [mol/mol]
needed to provide sufficient liquid to maintain a uniform suspension of the biocatalyst
(Hernandez-Martin et al., 2008). In addition, as Novozym 435 is a robust biocatalyst in the
presence of short chain alcohols, the excess alcohol kept the glycerol in solution, which
prevented deactivation of Novozym 435 by glycerol blockage of the entrance to catalyst
pores (Hernandez-Martin et al., 2008). Therefore, the levels of Novozym 435 used and the
stepwise methanol addition seem to avoid the possible diffusion limitations, which resulted
in high production yields.
4.5
40.0
80.0
4.0
3.5
3.0
2.5
2.0
60.0
20.0
1.5
5.0
7.5
10.0
12.5
15.0
Novozym 435 [wt%]
B
Fig. 3. Contour plots of FAME production yield predicted from the model for 50% (wt) WFO
at (A) a methanol-to-oil ratio of 3:1 (mol/mol) and (B) 45ºC.
Properties of feedstock that affect methanolysis optimization. The feedstock characteristics were
investigated to determine the components responsible for the results obtained in the RSM,
particularly the increase in the FAME yield when WFO was incorporated in the process
(Table 2).
Differences were found in most of the physical properties of the oil feedstock measured
(Table 2). The food frying process produced an increase in both acid and peroxide values
from WFO due to the hydrolysis and oxidation reactions, respectively (Araújo 1995). The
high peroxide value of the WFO could positively affect the FAME characteristics by
increasing the oxygen content, which enhances its burning efficiency (Lin et al., 2007).
However, a higher oxygen content in oils may also promote a higher nitrogen oxide
concentrations during the burning process (Lin et al., 2007).
The analyzed WFO was originally an edible soybean oil and sunflower oil mixture, which is
characterized by a typical iodine value of about 130 g I2/100 g oil. As the iodine value is
diminished by polymerization reactions during the frying process, a similar iodine value
compared to rapeseed oil was found for WFO (Table 2). A lower iodine value increases the
pour point and may also improve the oxidative stability of the oil (Canakci 2007). However,
a high pour point can negatively affect FAME performance in an engine, when used at low
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
285
temperatures. The kinematic viscosity of WFO was also measured and was slightly higher
compared to rapeseed oil, probably as a result of polymerization reactions. As WFO is
refined oil, the sulfur content was lower in WFO compared to rapeseed oil, which is an
unrefined oil. In addition, a higher FFA content in WFO compared to rapeseed oil was
observed, which may enhance FAME production yield using a biological catalyst, such as
Novozym 435, through direct FFA esterification (Li et al., 2007).
Physicochemical properties
Feedstock
WFO
Rapeseed oil
Specific gravity [kg/m3 at 20°C]
925
927
Kinematic viscosity at 40°C [cst]
37
34
Acid value [mg KOH/g]
4.5
0.8
Iodine value [g I2/100 g oil]
111
105
Peroxide value [meq/kg]
15.2
4.1
Water and sediments [%v/v]
1.3
< 0.01
S [mg/L]
2.0
7.0
Pour point [°C]
-6.7
-23.3
Flash point [°C]
>100
>100
Palmitic [C16:0]
12.2
4.2
Stearic [C18:0]
4.1
1.3
Oleic [C18:1]
28.5
66.6
Linoleic [C18:2]
49.5
18.7
Linolenic [C18:3]
3.5
7.7
Arachidic [C20:0]
0.2
0.4
Eicosenic [C20:1]
0.1
1.0
Fatty acid composition [wt%]
Eicosapentaenoic [C20:5]
0.2
0.1
Erucic [C22:1]
0.0
0.1
Others
1.7
0.0
0.1
0.0
Acylglycerols and FFA composition [wt%]
Monoacylglycerol
Diacylglycerol
3.3
0.5
Triacylglycerol
93.5
98.7
Free fatty acid
3.1
0.8
Table 2. Properties and composition of oil feedstocks (samples were filtered before to the
analysis).
286
Enzyme Inhibition and Bioapplications
The FFA esterification reaction produces 1 mol of water per 1 mol of FAME produced. In
this work, water and sediments were quantified and low levels were found in both rapeseed
oil and WFO, with a slightly higher water content for WFO (Table 2). However, as WFO has
a high FFA content, incorporation of WFO in the mixed feedstock may increase the water
content through FFA esterification reactions. Although lipases need an optimal amount of
water to maintain their activity in organic media, a recent work established that Novozym
435 itself appears to contain sufficient water to preserve its catalytic activity without
requiring the presence of an oil/water interface (Ognjanovic et al., 2009). It has also been
shown that Novozym 435 needs a nearly anhydrous reaction medium to be effective (Salis et
al., 2005). According to these previous studies, when WFO was incorporated in a mixed
feedstock, the reaction yield diminished as a result of the high water content. Therefore, an
improvement in FAME production yield when WFO is incorporated into a feedstock should
not be produced due to the presence of water in the transesterification reaction.
Differences between the fatty acid compositions of WFO and rapeseed oil were also detected
(Table 2). In order to analyze the role of fatty acids in FAME production, the input and
output fatty acid compositions were compared. However, no significant differences between
input and output fatty acid composition were detected. (data not shown). Products from oil
conversion, i.e., MG, DG, TG and FFA, in the used feedstock were compared using GC-MS.
MG, DG and FFA account for about 6.5% of the WFO composition, compared to only 1.3%
in the case of rapeseed oil (Table 2). Turkan & Kalay (2006) established that in Novozym
435-catalyzed transesterification, the first step (i.e., conversion of TG to DG) is the ratedetermining step, since TG is converted to FAME without significant accumulation of MG
and DG. This fact indicates that MG, DG and FFA can be more easily converted to FAME
compared to TG. Therefore, WFO is a more available substrate in the process investigated,
compared to rapeseed oil, and incorporation of WFO in feedstock increased the FAME
production yield.
Finally, is possible to establish that according to the RSM, the optimized predicted
combination of conditions to obtain 100% FAME production yield was the use of 100% (wt)
WFO, a methanol-to-oil ratio of 3.8:1 (mol/mol), 15% (wt) Novozym 435 and 44.5°C at 200
rpm. Methanol addition in two steps was previously optimized. An earlier second addition
of methanol after 8 hours of reaction time was an effective technique to decrease the total
reaction time to 12 hours, while preventing enzyme inhibition. The model obtained from the
RSM predicted that several optimal conditions were able to reach the highest FAME
production yield. According to this model, the addition of WFO increased the FAME
production yield, and this effect was mainly attributed to the higher contents of MG, DG
and FFA of WFO compared to rapeseed oil, which are more available substrates for
enzymatic catalysis. Therefore, a partial replacement of rapeseed oil by WFO in processes
catalyzed by Novozym 435 may diminish the cost of biodiesel production by using a less
expensive feedstock that simultaneously increases the production yield.
3. Enzymatic biodiesel production in an anhydrous medium with lipase
reutilization
A major problem in lipase-catalyzed processes to FAME production is the high acidity of the
final product, mainly caused by water presence. In fact, water favors the hydrolysis reaction
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
287
producing FAME with a high acid value. Therefore, to accomplish with international biofuel
standards, additional treatment are needed (Fukuda et al., 2008). In addition, this problem
could be enhanced by using alternative feedstock characterized by high acid value, such
WFO, jatropha, microalgae and crude palm oil (Azócar et al., 2010b).
To solve these drawbacks the addition of water adsorbents to the lipase-catalyzed reaction
has been recently investigated (Li et al., 2009; Wang et al., 2006). Some reported absorbents
are blue silica gel, maceo-pored silica gel, fine-pored silica gel, and molecular sieves of 3 Å, 4
Å and 5 Å (Li et al., 2009; Wang et al., 2006). Among the adsorbents examined, blue silica gel
has been reported as the best one, increasing FAME yield when immobilized lipase from
Penicillium expansum was used as catalyst (Li et al., 2009). However, high dosage of silica gel
could provoke low biodiesel yield. In fact, as the pore size of silica gel is much larger than
the methanol molecule size, silica adsorbs methanol negatively affecting the reaction
performance (Wang et al., 2006).
The possible advantages of using water adsorbents in lipase-catalyzed processes to improve
FAME production yield may be in contradiction with reports showing that lipases activation
may need water presence. In fact, lipases activation involves the active site restructuration,
which requires the presence of an oil-water interface (Yu et al., 2010). As a result, small
dosages of water in the reaction have been proposed to achieve an effective process (Yu et
al., 2010). Novozym 435 has been shown to contain enough water in its support medium to
preserve its catalytic activity in an anhydrous medium (Ognjanovic et al., 2009;
Tamalampudi et al., 2008). In a response surface methodology study, Organovic et al. (2009)
correlated Novozym 435 concentration with water concentration, achieving the highest
biodiesel yield of 92% using the medium with the lowest water content level (0%) and the
highest enzyme concentration (5% based on oil weight). In another study, Novozym 435
showed low activity in the presence of water and an anhydrous medium for efficient
catalyzing was proposed (Tamalampudi et al., 2008).
Novozym 435 has been widely reported as an effective biocatalyst for promoting high
FAME production yields (Azócar et al., 2010b). Optimal conditions to reach 100% FAME
yield using WFO as feedstock were determined to be the following: Methanol-to-oil molar
ratio 3.8:1 (wt), 15% (wt) Novozym 435 and incubation at 44.5°C for 12 h with agitation at
200 rpm (Azócar et al., 2010a). Novozym 435 activity loss and regeneration have been also
investigated. It has been established that the immobilization material (acrylic resin) could
adsorb polar components such as methanol and glycerol, provoking enzyme inactivation.
Therefore, Novozym 435 reutilization by means of washing processes using acetone,
soybean oil, tert-butanol, isopropanol and 2-butanol has been investigated (Chen et al., 2003;
Samukawa et al., 2000). In addition, the incorporation of a co-solvent in the reaction has
been recently proposed (Royon et al., 2007; Yu et al., 2010).
According to previous research, Novozym 435 has been proved to be an effective catalyst in
FAME production and has been shown to preserve its catalytic activity in an anhydrous
medium. However, Novozym 435 has not been investigated yet regarding catalyzedprocesses when adsorbent materials are added to the reaction in order to obtain high quality
biodiesel with low acid value. In this sense, it is necessary to evaluate the behavior of the
enzymatic reaction in anhydrous medium. In addition, it is necessary to determine the effect
288
Enzyme Inhibition and Bioapplications
of water absence in the enzyme activity during consecutive reactions and to evaluate
alternatives for enzyme recovery during successive reactions.
The aim of this work was to test an anhydrous medium in Novozym 435 catalyzed-process
to produce FAME with a low acid value. In addition, the behavior of Novozym 435 in the
anhydrous medium was discussed and enzyme recovery alternatives were proposed.
Filtered WFO collected from restaurants and crude rapeseed oil from a local factory in
Southern Chile were used as feedstock. The WFO was characterized by an acid value of 5.61
mg KOH/g oil, 2.8% FFA, 1.3% (v/v) water content, 0.1% (wt) MG, 3.3% (wt) DG and 925
Kg/m3 of specific gravity. The rapeseed oil was characterized by an acid value of 0.8 mg
KOH/g oil, 1.6% FFA and 927 Kg/m3 of specific gravity. Novozym 435 donated by Novo
Industries (Denmark) was used as catalyst. 3 Å molecular sieves (Sigma-Aldrich) were used
to generate the anhydrous medium. Chromatographically pure methyl heptadecanoate,
1,2,3-butanetriol and 1,2,3-tricaprinoylglycerol were used as internal standards.
FAME production using Novozym 435 in an anhydrous medium was studied by adding 3 Å
molecular sieves to the reaction. All reactions were carried out in flasks containing 1 mL of
WFO (0.925 g) and 15% Novozym 435 (% wt based on oil weight) as the biocatalyst. A
methanol-to-oil molar ratio of 4:1 was used. Methanol was added in two steps in order to
avoid lipase inhibition (according to previous experiments in section 2). The flasks were
incubated in a shaker at 35 °C and stirred at 200 rpm. Each flask was managed as a
destructive sample at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 hours of reaction time and each
condition was carried out in triplicate. Controls were carried out under the same conditions
but without molecular sieves.
To calculate the dosage of molecular sieves, first the maximal theoretical water content in
the reaction was estimated. This value was established by taking the initial water content in
the WFO and the amount of water produced during the reaction by esterification of FFA
contained in the WFO. The water produced by esterification of FFA was estimated by using
the mass balance, considering that 1 mol of esterified FFA produces 1 mol of water. With an
FFA content of 2.8% and water content of 1.3% in the WFO described above, a total water
content of 4.1% was used to estimate the molecular sieve dosage in the reaction, as shown in
Eq. 3.
Molecular sieves 
V oil   oil  H 2O 1  0.925  4.1

 0.19 g
20
WAC
(3)
Where Voil (mL) is the volume of oil added, ρoil (g/mL) is the density of the oil, H2O is the
total water content during the reaction (water content in the oil more water produced by
esterification of FFA) (% v/v) and WAC (%) is the water absorption capacity of the sieves.
At the end of the reaction period, the samples were stored at 4 °C to stop the reaction. The
samples were centrifuged and the upper layer was extracted for the analysis of FAME yield
and acid value.
Different treatments to recover enzyme activity for further reutilization were investigated.
Prior to the enzyme recovery treatments, reactions of FAME production were carried out in
flasks which were incubated in a shaker under the same operating conditions. The
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
289
operational conditions were the following: Methanol-to-oil ratio of 3.8:1 (mol/mol); 15%
Novozym 435 (% wt based on oil weight); 44.5 ºC; stirring at 200 rpm; and 12 hours of
reaction time. Methanol was added in two steps as described previously.
For each reaction, 8 mL of WFO (7.4 g) with an acid value of 5.61 mg KOH/g oil were
used. To maintain an anhydrous medium, 0.95 g of 3 Å molecular sieves were added to
each reaction flask, which were calculated according to Eq. (3). Each reaction was carried
out in triplicate. Samples of 70 µL were taken consecutively at 3, 6, 9 and 12 hours of
reaction time. The samples were centrifuged and the upper layer was extracted to analyze
the FAME yield.
For enzyme recovery, the molecular sieves were separated with a strainer after the
transesterification reaction. A paper filter over a vacuum system was placed underneath the
strainer containing the molecular sieves to retain the enzymes. The product of the reaction
was passed through the two filters: The molecular sieves captured in the strainer were
eliminated, whereas the enzymes captured in the paper filter were placed in a new flask for
the recovery treatment.
Three treatments to reutilize the enzymes were performed according to the reference studies
(Chen et al., 2003; Samukawa et al., 2000; Yu et al., 2010) and a previous experiments (data
not shown). A control experiment reusing the enzymes without treatment was also carried
out. The treatments were the following: (A) Acetone washing: The enzymes were washed by
adding a small dosage of acetone to the flask containing the enzymes. The flask was shaken
and the liquid residue was eliminated. This was repeated successively until the liquid was
clear. The total volume used in this process was about 10 mL of acetone per gram of
enzyme. Subsequently, a wash with 10 mL of WFO per gram of enzyme was carried out to
eliminate residual acetone in the enzymes. (B) Waste frying oil washing: The enzymes were
washed in a sufficient quantity of WFO to maintain the enzymes submerged in the flask.
The flask was shaken and the liquid residue was eliminated. The washing was repeated 3
times. (C) tert-Butanol washing: The washing was carried out by adding a specific dosage of
tert-butanol to the flask containing the enzymes. Then, the mixture was shaken and the
residual liquid was eliminated. This sequence was repeated consecutively until the liquid
was clear. The total volume used in this process was about 10 mL of tert-butanol per gram of
enzyme. After this, a wash with about 10 mL of WFO per gram of enzyme was carried out to
eliminate residual tert-butanol in the enzymes.
After each washing step, a known dosage of WFO was added to the flasks containing the
enzymes, which were incubated in oil overnight at ambient temperature. After about 10
hours, both methanol and 3 Å molecular sieves were added to carry out a new FAME
production reaction in the same incubation medium. The control was carried out by
transferring the enzymes directly to a new reaction for FAME production. Cycles that
consisted of both a reaction for FAME production and a treatment for enzyme recovery
were repeated 4 times.
As an alternative to enzyme reuse for FAME production using an anhydrous medium, the
addition of tert-butanol as a co-solvent in the reaction was investigated. To carry out the
experiments, consecutive cycles of enzymatic FAME production using the same enzymes
(reutilized) were carried out in an anhydrous medium and in a tert-butanol system. Each
290
Enzyme Inhibition and Bioapplications
experiment was carried out in flasks by adding 5 mL of WFO (4.6 g), 15% of Novozym 435
(% wt based on oil weight) and a methanol-to-oil ratio of 3.8:1 (mol/mol). Methanol was
added to the flask in two steps, similar to the previous experiment. In addition, both the
selected dosage of tert-butanol and an amount of 3 Å molecular sieves (estimated according
to Eq. 1) were added to the flasks. The tert-butanol dosage was established according to
previous experiments at 0.75% (V/V) (data not show). Under these conditions, the flasks
were incubated at 44.5 °C and 200 rpm during 12 hours of reaction time. All the experiments
were carried out in triplicate and samples were taken throughout the reaction time. The
upper layer of the centrifuged samples was analyzed to determine the FAME yield,
according to the analytical procedures.
The acid value and FFA content in both the feedstock and throughout the reaction time were
determined by titration with a KOH solution of known concentration and using
phenolphthalein as indicator. Water and sediments were measured according to ASTM
Standard Method D 1976-97(American). FAME, MG and DG in feedstock and reaction
products were quantified according to methodology previously described in section 2.
Biocatalysis in an anhydrous medium. In order to reduce the acid value of the produced FAME,
the alternative of carrying out FAME production reaction in an anhydrous medium was
studied. Adsorbent materials such as blue silica gel and molecular sieves of different sizes
have been shown to be effective in water removal during biodiesel production (Li et al.,
2009). However, in this study, blue silica gel was discarded due to results from previous
experiments showing possible methanol adsorption, negatively interfering with the reaction
(data not shown). The adsorption of methanol could also occur when molecular sieves with
large pore size are used. Thus, 3 Å molecular sieves were chosen to produce an anhydrous
medium through water absorption.
The acid value and FAME yield results obtained using an anhydrous medium and a control
run are shown in Fig. 4. The acid value obtained from the reaction using the anhydrous
medium was maintained in about 1 mg KOH/g oil throughout the entire reaction time,
whereas the control showed a value higher than 3 mg KOH/g oil by the end of the reaction
(Fig. 4A). The values obtained using the anhydrous medium are close to those established
by the biodiesel norm. As FFA present in WFO were esterified producing water and FAME
at the start to the reaction (Fig. 4A), water was removed from the reaction by adsorption
onto the molecular sieves, avoiding hydrolysis reactions and further FFA production.
Therefore, the biocatalysis in an anhydrous medium produces a higher quality product
compared to a typical standard reaction using Novozym 435.
FAME yield was also analyzed in an anhydrous medium (Fig. 4B). The results obtained
show that water removal during FAME production generated a significant increase in
FAME yield using Novozym 435 as the catalyst. These results are in agreement with those
obtained by Li et al. (2007) who established that using 3 Å molecular sieves as adsorbent to
remove excessive water could significantly increase the FAME yield when 100% FFA is used
as raw material for biodiesel production. In this study, WFO containing only 2.8% FFA and
1.3% water was employed as the raw material, but FAME yield increased drastically. In
addition, the control run only using molecular sieves indicates that this material did not
catalyze the reaction (data not shown). Therefore, the increment in FAME yield is only
related to the anhydrous medium. This corroborates the results obtained by Tamalampudi
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
291
et al. (2008) who suggest that Novozym 435 transesterification activity is inhibited by the
presence of added water and that it needs a nearly anhydrous medium for an efficient
performance.
7
A
Co ntr ol
Anhy dro us m edium
6
5
4
3
2
1
0
0
2
4
6
100
8
10
12
B
Control
Anhydrous medium
80
FAME content [%]
[h]
60
40
20
0
0
2
4
6
8
10
12
time [h]
Fig. 4 Acid values (A) and FAME yield (B) during the reaction in an anhydrous medium
with a methanol-to-oil ratio of 4:1 (mol/mol), 15% (wt) Novozym 435 (based on the weight
of oil), 35 ºC, 200 rpm and 12 hours of reaction time.
292
Enzyme Inhibition and Bioapplications
A reasonable explanation for the high FAME yield obtained may be related to the work of
Cabrera et al. (2009). They suggest that lipase may exist in two different structural forms; in
one form, the site of the lipase is isolated from the medium whereas in the other form, the
active site is exposed to the reaction medium. In a homogeneous aqueous medium the lipase
is in equilibrium between these two structures. In the case of Novozym 435, it is
immobilized in acrylic resin with hydrophilic properties. Therefore, the higher yield in the
reaction may occur because of interactions with the hydrophobic medium, where the lipase
shifts towards the open structure form, increasing its activity. These conformational changes
enable lipases to be greatly altered by controlled immobilization of the support material
properties. This is in accordance with the results obtained by Samukawa et al. (2000) who
established that preincubation of the enzyme in oil prior to the reaction improves the yield.
This is because water adsorbed during the reaction prevented oil penetration, whereas this
did not occur with the preincubated enzyme.
Treatments for enzyme reutilization in an anhydrous medium. The main advantage of using
immobilized lipases is that the enzyme can be used repeatedly in a semi-continuous process.
However, this objective is not always reached under the optimized conditions as short chain
acyl acceptors can produce a loss of enzyme activity in successive reactions (Ognjanovic et
al., 2009). In this study, the stability of the enzyme and treatments for its reutilization in an
anhydrous medium were examined. Fig. 5 shows the results obtained by different
treatments to maintain lipase activity after each FAME production reaction in an anhydrous
medium. Fig. 5A shows the reutilization of Novozym 435 without treatment after reaction.
According to the results obtained, FAME yield decreased considerably in the second cycle.
This tendency was maintained in successive cycles, with values close to 0% of FAME yield
in the fourth cycle. These results are in accordance with the findings of Ognjanovic et al.
(2009), who reported 0% FAME yield in the fourth cycle using Novozym 435 and methanol
as the acyl acceptor in an organic medium. Thus, although the activity of the enzyme
increased in the first cycle using an anhydrous medium compared to the control (Fig. 4), the
loss of activity seems to be similar in both types of medium when successive cycles are
carried out (Fig. 5A). Lipases have shown high synthesis activity and stability in
hydrophobic solvents, but alcohol and glycerol are immiscible in those solvents (Halim et
al., 2008). This situation could produce poor solubilization of polar compounds in the
medium, leading to their adsorption onto the lipases hydrophilic support and therefore
provoking a low transesterification rate (Halim et al., 2008). So far, an alternative process is
needed to allow the reuse of the enzyme achieving a low-cost process which can feasibly be
implemented at an industrial scale.
The results of the first treatment studied for reusing the enzyme are shown in Fig. 5B.
Applying an acetone washing step a higher FAME yield was achieved in the second cycle
compared to the control (Fig. 5B and 5A, respectively). This result is caused by the fact
that acetone is a hydrophilic solvent that could remove the glycerol and the methanol
adsorbed onto the hydrophilic support material of the enzyme. However, in the third
cycle the FAME yield diminished drastically (Fig. 5B). Acetone is a hydrophilic solvent
with a very low log P (partition coefficient in a standard octanol-water two-phase system)
value of -0.24. According to Yu et al. (2010) water has higher affinity to these hydrophilic
solvents rather than to the enzyme. It has been reported that Novozym 435 can contain a
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
293
sufficient quantity of water to preserve its catalytic activity in an anhydrous medium
(Ognjanovic et al., 2009; Tamalampudi et al., 2008). Therefore, the enzyme activity may be
reduced when using an acetone washing step, because the enzyme might lose its
flexibility conformation due to the lack of bound water (Yu et al., 2010). The enzyme
washing step using WFO followed by overnight incubation in WFO was also studied (Fig.
5C). The activity of Novozym 435 was higher in the third cycle in comparison to the two
previous alternatives. Samukawa et al. (2000) reported that Novozym 435 preincubated in
methyl oleate and subsequently in soybean oil could enhance the rate of FAME
production through impregnation of these compounds into the enzyme support material.
In this study, it was assumed that the enzyme could contain methyl ester residues when
incubated in WFO and therefore, the treatment should be similar to that carried out by
Samukawa et al. (2000).
A
60
40
B
100
FAME content [%]
80
20
80
60
40
20
0
0
0
10
20
30
40
time [h]
50
0
C
10
20
80
60
40
20
30
40
time [h]
100
FAME content [%]
100
FAME content [%]
FAME content [%] FAME content [%]
100
50
D
80
60
40
20
0
0
0
10
20
30
time [h]
40
50
0
Time [h]
10
20
30
40
50
time [h]
Fig. 5. Comparison of reutilization treatments of Novozym 435 after the reactions to FAME
production with a methanol-to-oil ratio of 3.8:1 (mol/mol), 15% (wt) Novozym 435 (based
on the weight of oil), 44.5 ºC, 200 rpm and 12 hours of reaction time. A) Control, B) Acetone
washing, C) Waste frying oil washing D) tert-Butanol washing.
This treatment using WFO is advantageous compared to the acetone washing process
because it is a cheaper and more environmental friendly process. In addition, the same WFO
that was used for the incubation was used subsequently for the reaction of FAME
294
Enzyme Inhibition and Bioapplications
production, and therefore less equipment is needed to carry out this process. In another
study, Chen and Wu (2003) achieved a FAME yield five times higher when the enzyme was
incubated overnight in soybean oil. However, when they used the same oil in incubation
experiments to recover the enzyme after the reaction, the activity began to decay after the
fourth reutilization. Although the advantages of using WFO in enzyme recovery, FAME
yield also declined in this study, as reported by Chen and Wu (2003). The reason may be the
low solubility of alcohol and glycerol in the oil (hydrophobic wash), which impedes the
efficient washing of the enzyme by oil.
As the washing process with hydrophobic and hydrophilic solvents did not allow an
efficient recovery of the enzyme activity by more than three successive reactions in an
anhydrous medium, a moderate polar solvent was also investigated. Tert-butanol, a
tertiary alcohol with a log P value of 0.35 has been shown to improve enzyme activity
more than linear alcohols. This fact could be attributed to the differences in miscibility
with triglycerides, as compared to alcohols with the same carbon numbers, branched ones
have better miscibility with triglycerides compared to linear isomers (Yu et al., 2010).
Therefore, a washing treatment of the enzymes with tert-butanol, followed by incubation
in WFO, was conducted. The results showed that this enzyme pretreatment achieved the
best results, maintaining a higher activity over the time compared to the previous
experiments (Fig. 5D). It is likely that tert-butanol recovered the enzyme activity because
it has the advantages of both hydrophilic and hydrophobic solvents but none of the
drawbacks. Therefore, tert-butanol should promote the removal of both glycerol and
methanol from the lipase support material because of its hydrophilic properties, while its
hydrophobic properties should help maintain a high level of lipase activity. This is in
accordance to Chen and Wu (2003) who found that tert-butanol can be even used to
regenerate a deactivated, immobilized enzyme such as Novozym 435. Therefore, tertbutanol is a promising alternative for enzyme recovery after reaction in an anhydrous
medium, and is also highly stable and less reactive than other butanol isomers. Based on
these results, a new experiment was carried out which included tert-butanol as a cosolvent in the reaction in an anhydrous medium.
Successive reactions to FAME production in an anhydrous medium using tert-butanol as a cosolvent. A previous study found that tert-butanol is inert in the methanolysis system,
whereas it is also a potential co-solvent that could maintain enzymatic activity (Li et al.,
2006). According to this, the performance of tert-butanol as a co-solvent was studied in an
anhydrous medium to determine its ability to maintain Novozym 435 activity in successive
reactions. In Fig. 6 is shown the successive reactions for FAME production that were carried
out in an anhydrous medium under previously optimized operational conditions, using a
previously selected dosage of tert-butanol (0.75% v/v, data not show). A control without
tert-butanol as co-solvent was also carried out, showing that FAME yield was drastically
reduced in the second cycle (< 20% FAME yield). The best results were obtained in the
system using the co-solvent, where FAME yield was maintained over 50% after 17 cycles
(Fig. 6). According to similar findings from other studies, the enzyme is inhibited
throughout the reaction (Royon et al., 2007). The inhibitory effect at the beginning of the
reaction is due to the presence of methanol which has poor miscibility in oil. Subsequently,
once methanol concentration decreases, inhibition is caused by a glycerol layer coating the
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
295
catalyst. Therefore, the positive effect of tert-butanol in the reaction could be first related to
the increment in the oil-methanol miscibility, preventing direct contact between enzyme and
alcohol at the beginning of the reaction and improving the reaction yield. Secondly, due to
tert-butanol’s hydrophilic properties, it is able to dissolve both methanol and glycerol
during the reaction, preventing enzyme inhibition throughout the reaction. Thirdly, tertbutanol’s hydrophobic properties maintained the high lipase activity through the successive
reactions.
Anhydrous medium with tert-butanol
Control
FAME content [%]
100
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
time [h]
Fig. 6. Time course of FAME yield during 17 batch reaction cycles with a methanol-to-oil
ratio of 3.8:1 (mol/mol), 15% (wt) Novozym 435 (based on the weight of oil), 44.5 ºC, 0.75%
v/v of tert-butanol, 0.9 g of molecular sieves, 200 rpm and 12 hours of reaction time.
Symbols: in tert- butanol system (closed circles), control (open circles).
According to Yu et al. (2010), different properties such as viscosity, dielectric constant,
solubility parameters and log P should be considered when choosing a co-solvent to
improve enzyme performance in biodiesel production. Another aspect which should be
considered is the feasibility of recovering the co-solvent through distillation, which is
related to the boiling point. In this sense, tert-butanol has advantages compared to others cosolvents reported, such as amyl alcohol, which has very similar properties compared to the
tert-butanol but a higher boiling point (102 °C) close to the boiling point of the water. As the
boiling point of tert-butanol is 82 °C and the distillation of methanol occurs at 65 °C, the
recovery of tert-butanol should not increase the amount of energy spent that much in
comparison to the advantages of using this co-solvent. Therefore, employing tert-butanol as
a co-solvent could allow the use of an anhydrous medium as an industrial alternative for
FAME production using Novozym 435 as biocatalyst.
296
Enzyme Inhibition and Bioapplications
The results of this section showed that the use of an anhydrous medium (resulting from
water extraction by using molecular sieves), FAME yield was improved by avoiding
hydrolysis and esterification reactions, producing FAME mainly through transesterification
of TG. The enzyme activity cannot be recovered successively neither using a hydrophilic
washing step with acetone nor by hydrophobic washing with WFO. However, 17 successive
cycles of FAME production using tert-butanol as a moderate polar co-solvent, show that
Novozym 435 can be reused in anhydrous medium. These results also show that the
anhydrous medium could enable the implementation of lipase-catalyzed processes on an
industrial scale for biodiesel production mainly by transesterification reaction. Different raw
materials could be used for this, achieving properties close to the norm and potentially
avoiding post-treatments to refine the produced biodiesel.
4. Enzymatic transesterification reaction kinetic in a tert-butanol system for
biodiesel production
For the design of suitable reactors to FAME production using lipases, kinetic information of
the rate of product formation and the effects of changes in system conditions is needed.
Investigation about kinetic of FAME production using different lipases has been recently
reported. The first researches were focused in the esterification of FFA, where the Ping Pong
model with competitive inhibition by methanol was used to describe the reaction (Krishna
et al., 2001). After, Al-Zuhair et al. (2007) proposed a kinetic model based in the
transesterification of TG. The aim of this work was study the kinetic of the
transesterification to FAME production catalyzed by Novozym 435. The investigation was
carried out in anhydrous medium, using WFO as raw material, methanol as acyl acceptor
and tert-butanol as co-solvent. In addition, the set-up of a semi-continuous enzymatic
bioreactor was carried out. To realize the study Ping Pong model with competitive
inhibition by methanol was used (Eq. 4).
Vmax

1
KW 
[M]  KM
1 

[W ] 
K IM  [ M ]
(4)
Where υ (mol/L/min) is the initial reaction rate, Vmáx is the maximum rate of reaction
(mol/L/min), KW and KM are the binding constants for the WFO (W) and the methanol (M)
(mol/L), [W] is WFO concentration (mol/L), [M] is methanol concentration (mol/L) and KIM
is the inhibition constant for the methanol (mol/L).
To determinate the sole effect of methanol in the transesterification, the experiments was
run using methanol concentrations in the range of 100-3000 mol/L, equivalent at methanol
to oil molar ratio 0.6-15 (mol/mol), at a constant WFO concentration of 300 mol/L. The
WFO concentration was chosen to ensure operating outside WFO limitation or inhibition
region. To determinate the sole effect of WFO in the transesterification , the experiments was
run using WFO concentrations in the range of 200-350 mol/L, at a constant methanol
concentration of 1400 mol/L. The methanol concentration was chosen to ensure operating
outside alcohol limitation or inhibition region. Michaelis-menten kinetic was used to
estimate the kinetic constants Vmáx, KW y KM. Subsequently, the values were optimized and
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
297
the kinetic constant KIM was obtained using Excel solver to find the minimum objective
function that compares the measured rate of the reaction with those predicted by the
proposed kinetic equation. Each assay were performed in quadruplicate in Erlenmeyer flask
of 25mL and incubated in an orbital shaker during 4 hr under the same operational
condition: 15% (wt) of Novozym 435, 44°C, 0.75% (V/V) of tert-butanol, 0.5 g of molecular
sieves and 200 rpm. In order to remove the tert-butanol, the samples were centrifuged and
heated at 85°C during 30 minutes. The supernatant was used to quantify FAME yield using
GC-MS methods.
-1
-1
Simulate initial rate of reaction, u [mol L min ]
Using the experimental results the kinetic parameters estimated for the model were: Vmáx=
0.018 mol/L/min, KM, metanol = 1030 mol/L, KW, WFO= 397 mol/L y KIM, metanol= 1,815 mol/L.
The Fig. 7 shows a comparison between the model predictions and the experimental data of
the initial rate of reaction at different methanol concentration using Novozym 435.
0.005
0.004
0.003
0.002
0.001
0.000
0.000
0.001
0.002
0.003
0.004
-1
-1
Experimental initial rate of reaction, u [mol L min ]
0.005
Fig. 7. Comparison between the experimental results and the Ping-Pong kinetic model
equation with the estimated constants to different initial methanol concentration and initial
WFO concentration of 300 mol L-1.
298
Enzyme Inhibition and Bioapplications
According to Fig.7 the kinetic model is suitable to predict the behavior of the reaction,
indicating that the use Michaelis-Mentel kinectic could simplify the calculation of kinectic
constant of complex model such as Ping Pong kinetic model. The model and empirical data
showed that Novozym 435 presented a low inhibition by methanol, even at higher
concentration maintaining a high initial reaction rate until a methanol to WFO molar ratio of
8/1 (mol/mol). The model predicts a moderate decrease in the initial reaction rate for higher
concentration of methanol. This behavior could be associated to the incorporation of the cosolvent in the reaction due its capacity to improve the miscibility of reaction mix. This is an
advantage because higher concentration of methanol favor the production formation since
the transesterification is an equilibrium reaction, decreasing the operation time of the FAME
production using enzymatic catalysts.
100
FAME content [%]
80
60
40
20
0
0
0
1
2
10
3
4
Cicle [N°]
20
5
30
6
7
40
50
time [h]
Fig. 8. FAME yield during the set-up of a semi-continuous enzymatic reactor in anhydrous
medium with enzymes reutilization. Operational conditions: methanol-to-oil ratio of 8/1
(mol/mol), 15% (wt) Novozym 435 (based on the weight of oil), 44.5 ºC, 0.75% v/v of tertbutanol, 200 rpm and 4 hours of reaction time.
Feasible Novozym 435-Catalyzed Process to Fatty Acid Methyl
Ester Production from Waste Frying Oil: Role of Lipase Inhibition
299
The results of the kinetic study were applied in the operation of a semi-continuous reactor to
enzymatic FAME production. The bioreactor was made of glass with a volume of 0.5 L. The
temperature was controller by a thermostat. The agitation of the bioreactor was supply by a
magnetic stirrer. In order to extract water constantly from media, a glass column filled with
molecular sieves was connected to the bioreactor, where the media was recycling using a
peristaltic pump. The glycerol was removed from bottom by a settler connected to the
bioreactor. The operational conditions to each reaction cycle were: methanol to oil molar
ratio 8/1 (mol/mol), 15 % (wt) Novozym 435, 0.75% (v/v) of tert-butanol, 44.5 °C, 200 rpm
and 4 hours of reaction time. The enzymes were reused successively remaining into the
reactor during all the cycles. In Fig. 9 are showed the results obtained.
The bioreactor was operated several cycles of charge and discharge during 30 h, adding
only raw material and changing the molecular sieves to maintain an anhydrous media.
Under these conditions was possible to maintain FAME yields over 80% during 7 reaction
cycles (Fig. 8). Therefore, the use of both an anhydrous media and tert-butanol as cosolvent are effective strategies for the implementation of a semi-continuous enzymatic
process that can become economically competitive with traditional chemical catalysts to
produce FAME.
5. Conclusions
The short reaction time of the proposed process and the reuse of the enzymes generate a
feasible alternative to be implemented at industrial scale. This process has several
advantages compared to the chemical process. Although the chemical process requires
only 1 hour of reaction time, after the process a washing step to remove catalyst residues
is necessary. This washing step is not necessary in the enzymatic process because
Novozym 435 is a solid catalyst. In addition, the enzymatic process has the advantage to
be flexible allowing the use of alternative and low cost raw materials. Therefore, the
results obtained in this work have generated an enzymatic process that could become not
only environmentally friendly but also economically competitive with the chemicalcatalyzed process.
The kinetic study and the reactor operation showed that Novozym 435 was not inhibited to
high methanol concentrations and could be reused without significant loss of activity. These
results shown that under the conditions investigated enzymatic FAME production could be a
competitive process to be implemented to industrial scale. To reach this objective is necessary
to carry on the bioreactor operation in the long time, in order to establish the lifespan of the
enzymes. In addition is necessary to establish a molecular sieves recuperation protocol and to
looking for inexpensive material such as zeolites to decrease the operational cost.
6. Acknowledgement
Research described was supported by Desert Bioenergy S.A., Chilean project “Inserción de
Capital Humano Avanzado en el Sector Productivo Chileno 78110106”, Chilean Fondecyt
project 3120171, Chilean CONICYT Project 79090009 and FONDECYT iniciación project
11110282.
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Enzyme Inhibition and Bioapplications
7. References
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biodiesel from palm oil using lipase. Process Biochem, 42, 951-960.
American Society for Testing and Materials, ASTM (2008): Annual Book of ASTM Standards:
Section 5 - Petroleum Products, Lubricants, and Fossil Fuels. ASTM International, West
Conshohocken.
Araújo, J. (1995). Oxidação de Lipidios, p. 1- 64. In Imprensa Universitária (ed.), Química de
alimentos: Teoría e prática. Universidad Federal de Viçosa. Viçosa.
Azócar, L., Ciudad, G., Heipieper, H., Muñoz, R. & Navia, R. (2010a). Improving fatty acid
methyl ester production yield in a lipase-catalyzed process using waste frying oils
as feedstock. J Biosci Bioeng, 109, 609-614.
Azócar, L., Ciudad, G., Heipieper, H. & Navia, R. (2010b). Biotechnological processes for
biodiesel production using alternative oils. Appl Microbiol Biotechnol, 88, 621–636.
Cabrera, Z., Fernandez-Lorente, G., Fernandez-Lafuente, R., Palomo, J. & Guisan, J. (2009).
Novozym 435 displays very different selectivity compared to lipase from Candida
antarctica B adsorbed on other hydrophobic supports. J Mol Catal B: Enzym, 57, 171–
176.
Canakci, M. (2007). The potential of restaurant waste lipids as biodiesel feedstocks. Bioresour
Technol, 98, 183–190.
Chen, J. & Wu, W. (2003). Regeneration of immobilized Candida antarctica lipase for
transesterification. J Biosci Bioeng, 95, 466-469.
Dizge, N., Aydiner, C., Imer, D. Y., Bayramoglu, M., Tanriseven, A. & Keskinlera, B. (2009).
Biodiesel production from sunflower, soybean, and waste cooking oils by
transesterification using lipase immobilized onto a novel microporous polymer.
Bioresour Technol, 100, 1983-1991.
Du, W., Xu, Y., Liu, D. & Zeng, J. (2004). Comparative study on lipase-catalyzed
transformation of soybean oil for biodiesel production with different acyl
acceptors. J Mol Catal B: Enzym, 30, 125-129.
Fukuda, H., Hama, S., Tamalampudi, S. & Noda, H. (2008). Whole-cell biocatalysts for
biodiesel fuel production. Trends Biotechnol, 26, 668-673.
Fukuda, H., Kondo, A. & Noda, H. (2001). Review: Biodiesel fuel production by
transesterification of oils. J Biosci Bioeng, 92, 405-416.
Gui, M., Lee, K. & Bhatia, S. (2008). Feasibility of edible oil vs. non-edible oil vs. waste edible
oil as biodiesel feedstock. Energy, 33, 1646-1653.
Halim, S. & Kamaruddin, A. (2008). Catalytic studies of lipase on FAME production from
waste cooking palm oil in a tert-butanol system. Process Biochem, 43, 1436–1439.
Hernandez-Martin, E. & Otero, C. (2008). Different enzyme requirements for the synthesis
of biodiesel: Novozym (R) 435 and Lipozyme (R) TL IM. Bioresour Technol, 99,
277-286.
Issariyakul, T., Kulkarni, M., Meher, L., Dalai, A. & Bakhshi, N. (2008). Biodiesel
production from mixtures of canola oil and used cooking oil. Chem Eng J, 140, 7785.
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Kose, O., Tuter, M. & Aksoy, H. (2002). Immobilized Candida antarctica lipase-catalyzed
alcoholysis of cotton seed oil in a solvent-free medium. Bioresour. Technol., 83, 125129.
Krishna, S. H. & Karanth, N. G. (2001). Lipase-catalyzed synthesis of isoamyl butyrate - A
kinetic study. Biochimica Et Biophysica Acta-Protein Structure and Molecular
Enzymology, 1547, 262-267.
Li, L., Du, W., Liu, D., Wang, L. & Li, Z. (2006). Lipase-catalyzed transesterification of
rapeseed oils for biodiesel production with a novel organic solvent as the reaction
medium. J Mol Catal B: Enzym, 58-62.
Li, N., Zong, M. & Wu, H. (2009). Highly efficient transformation of waste oil to biodiesel by
immobilized lipase from Penicillium expansum. Process Biochem, 44, 685-688.
Li, W., Du, W. & Liu, D. (2007). Rhizopus oryzae IFO 4697 whole cell-catalyzed methanolysis
of crude and acidified rapeseed oils for biodiesel production in tert-butanol system.
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Lin, C. & Lin, H. (2007). Engine performance and emission characteristics of a three-phase
emulsion of biodiesel produced by peroxidation. Fuel Process. Tech., 88, 35-41.
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sunflower oil to biodiesel in a solvent-free system: Process optimization and the
immobilized system stability. Bioresour Technol, 100, 5146-5154.
Royon, D., Daz, M., Ellenrieder, G. & Locatelli, S. (2007). Enzymatic production of
biodiesel from cotton seed oil using t-butanol as a solvent. Bioresour Technol, 98,
648–653.
Salis, A., Pinna, M., Monduzzi, M. & Solinas, V. (2005). Biodiesel production from triolein
and short chain alcohols through biocatalysis. J Biotechnol, 119, 291-299.
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Fukuda, H. (2000). Pretreatment of immobilized Candida antarctica lipase for
biodiesel fuel production from plant oil. J Biosci Bioeng, 90, 180-183.
Shimada, Y., Watanabe, Y., Sugihara, A. & Tominaga, Y. (2002). Enzymatic alcoholysis for
biodiesel fuel production and application of the reaction to oil processing. J Mol
Catal B: Enzym, 17, 133-142.
Tamalampudi, S., Talukder, M., Hama, S., Numata, T., Kondo, A. & Fukuda, H. (2008).
Enzymatic production of biodiesel from Jatropha oil: A comparative study of
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185-189.
The British Standards Institution (2003): BS EN 14214:2003. Automotive fuels. Fatty acid
methylesters (FAME) for diesel engines. Requirements and test methods, BSI, London.
Turkan, A. & Kalay, S. (2006). Monitoring lipase-catalyzed methanolysis of sunflower oil by
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Wang, L., Du, W., Liu, D., Li, L. & Dai, N. (2006). Lipase-catalyzed biodiesel production
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Watanabe, Y., Shimada, Y., Sugihara, A. & Tominaga, Y. (2001). Enzymatic conversion of
waste edible oil to biodiesel fuel in a fixed-bed bioreactor. J Am Oil Chem Soc, 78,
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10
Urease Inhibition
Muhammad Raza Shah1 and Zahid Hussain Soomro2
1International
Center for Chemical and Biological Sciences, H.E.J.
Research Institute of Chemistry, University of Karachi
2Institute of Materials Science and Research Dawood College
of Engineering and Technology Karachi
Pakistan
1. Introduction
The design, synthesis, characterization and exploring the broad spectrum of potency of
biologically active molecular building blocks have attracted the attention of scientific
community since last couple of decades. The emergence of pathogenic resistance is a natural
phenomenon and investigations toward the advances of new structurally diverse inhibitors
have always been at the esteem of pharmaceutical research. This growing field has ever
become an active area of research for the synthetic, biological and medicinal perspectives,
looking at the molecular level. Enzyme inhibition has attracted great attention of biomedical
scientist since last couple of decades. A variety of inhibitors have been discovered and used
for the control of various diseases. One of the key features of enzyme is its selectivity
towards certain inhibitors; the inhibitor can be a simple organic molecule or complex
molecular architecture. The specificity of inhibitor depends on the size, shape and the
interactive forces which result in the exact matching of inhibitor and the enzyme. These
inhibitions block the activity of the enzyme under the physiological conditions. Urea and
urease are the landmark molecules in the early days of synthetic organic chemistry. Urea
was the first organic molecule synthesized in laboratory and urease was being first
crystallized out from jack bean (Amtul et al., 2002; Hagar et al., 1925; Mobley et al., 1989;
Amtul et al., 2006; Zonia et al., 1995). The significance of the urease enzyme is critically
investigated due to its capacity to serve as a virulence factor of infections in the urinary and
gastrointestinal tracts, maintaining equilibrium in the nitrogen cycles of nitrogenous wastes
in the rumens of domestic livestock, and its importance in environmental transformations of
nitrogenous compounds, including urea based fertilizers. The physiology of microbial
ureases depends on their cellular location, regulation, genetic makeup and the relationships
between the microbial urease and the well-characterized potent range of inhibitors. The
urease (urea amido hydrolase; EC 3.5.1.5) is a nickel containing enzyme which hydrolyzes
urea into ammonia and carbamic acid, the later compound produce ammonia and carbon
dioxide on further decomposition. The hydrolysis of the urea yields high concentrations of
ammonia which accompanying in pH elevation. Urease has been regarded as the important
enzyme for the manipulation of urea and other nitrogenous ingredients in the biological,
agricultural and environmental fields. Urease occurs in many plants, selected fungi, and a
304
Enzyme Inhibition and Bioapplications
wide variety of prokaryotes. This enzyme is having important negative implications in
medicine, agriculture and environment. The urease inhibitors can play a vital role to counter
effect the negative role of urease in living organisms. Urease inhibitors are effective against
several serious infections caused by the secretion of urease by Helicobactor pylori which
includes gastric tract syndromes, proteus related species in the urinary tract, struvite
urolithiasis mainly in dogs and cats. The urease inhibitors have also received great attention
from scientists in the soil sciences whereas the nutrient miss-management is associated to
the excessive use of synthetic fertilizer and excess of urea products. Research on urease
inhibitions yielded several vital therapeutic drugs (Amtul et al., 2002). Hagar and Magath
have reported in 1925 that urease is the primary source for the biochemical formation of
stone in urinary tract (Hagar et al., 1925). Urease serves as virulence factor in the pathogens
responsible for kidney stones entailed in urolithiasis that contributes toward the acute
pyelonephritis with other urinary tract infection which induced arthritis and gastric
intestinal infections and ultimately the urease imbalance lead to peptic ulcers (Mobley et al.,
1989). Urease controls the nitrogen contents in the physiological systems, while it provides a
protection mechanism against predators and phytphathogenic organisms (Amtul et al.,
2006; Zonia et al., 1995). The high concentrations of ammonia arising from the biochemical
reactions of urease, as well as pH elevation result into important negative implications in
medicine and agriculture (Zonia et al., 1995; Collins et al., 1993; Montecucco et al 2001;
Zhengping et al). High concentrations of ureases cause significant environmental and
economic problems by releasing abnormally large amounts of ammonia into the atmosphere
during urea fertilization. It induces plant damage primarily by depriving plants of their
essential nutrients and secondarily by ammonia toxicity which result in pH increase of the
soil (Bremner et al., 1995). The urease inhibitors have played a vital role in the controlling
the Helicobacter pylori which caused an imbalance of ammonia levels in cirrhotic patients
(Zullo et al., 1998). The urease based liquid crystal-sensors are also used for the detection of
heavy metals; these urease based sensors are useful in monitoring the inhibition of
enzymatic activities that has been widely investigated for the detection of heavy metals
(HMs) (Verma et al., 2005). In many cases, after inhibition of immobilized enzymes with
HMs, chelating agents such as EDTA are usually used to regenerate the active site of the
enzyme (Bracka et al., 2000). Urease-based sensors, which are inexpensive and sensitive to
HMs, have been widely exploited in the scientific community (Volotovsky et al., 1997).
Several analytical methods for monitoring HMs are based on the enzyme-catalyzed
hydrolysis of urea into ammonia and carbon dioxide, as the consumption of urease
inhibitors is vital for the production of grains over the long-term, meanwhile the coating of
urea to reduce ammonia volatilization loss from urea fertilizer is gaining great attention of
researchers in order to control the depletion of active ingredients in the soil. The use of
fertilizer is very much common in the areas where the active ingredients do not occur in
sufficient quantities. The nitrogen losses can be reduced in these situations and the use of
urease inhibitor is applied to the fertilizer to maintain the optimum pH of the soil (Schwedt
et aj., 1993; Saboury et al., 2010). Recently, the LCs has been doped with functional
molecules that have been used to develop novel types of sensors. There are series of
compounds with 4-pentyl-biphenyl-4-carboxylic acid (PBA), which contains pH-sensitive
functional groups could be used in an LC-based pH sensor to monitor small amounts of H+
released from enzymatic reactions, especially in solutions with a high buffer capacity (Bi et
Urease Inhibition
305
al., 2009). Literature is rich on the role of H. pylori which is the main cause for the gastric
infections. Urease is a prominent antigen of the H. Pylori which has served as a powerful
immunogen for this organism. There have been various reports on large number of patients
who have shown gastritis significantly elevated response in immunoglobulins G and A
along with urease in the blood serum samples with relative comparison of pre-infected
levels. Range of enzymatic assays is reported to measure the immune responses and these
are used for the diagnostic purposes in monitoring the antibiotic therapy of H-Pylori for the
epidemiological studies (Amtul et al., 2002).
2. Inhibition mechanism
The inhibition mechanism is mainly based on the binding of the substrate and enzyme. The
urease has a bi metallic centric structure which binds with range of molecules depending on
the kinetics, dynamics, mechanism of action and the possible secondary interactions
involved in this phenomenon. The mechanism of inhibition can be categorized on the basis
of inhibitor, either as irreversible inhibitors or the reversible inhibitor where as the later one
is further classified into two categories based on the binding with enzyme either in a
competitive manner or in a non competitive manner. The process involved in case of
reversible inhibitors mostly occur through non covalent interactions such as hydrogen
bonding, hydrophobic interaction and orientation of inhibitor and enzyme in an organized
fashion (Amtul et al., 2002). The irreversible inhibitors interact through its functional groups
with amino acids in the active site, irreversibly e.g. the nerve gases and pesticides, which
contain an organophosphorus that bind with serine residues in the enzyme acetylcholine
esterase. The reversible inhibitors are of two types either as competitive or non-competitive.
The competitive inhibitors compete with the substrate molecules for the active site while the
inhibitor’s action is proportional to its concentration that resembles with the substrate’s
structure closely. The non-competitive inhibitors are not influenced by the concentration of
the substrate. It inhibits by binding irreversibly to the enzyme but not at the active site. The
non-competitor inhibitor follows the allosteric sites in the enzyme for the binding (Figure
2.1). The reversible inhibitors can be washed out of the solution of enzyme by dialysis.
Fig. 1. Model representation of the binding of the enzyme active site and inhibitor.
306
Enzyme Inhibition and Bioapplications
In the competitive inhibition the inhibitor directly binds to the active site of the enzyme via a
specific host and guest complex, while in the non-competitive mechanism the substrate
binds somewhere else and not specifically on the active site in the enzyme this binding of
substrate affect the structure of the enzyme and induce inhibition. Furthermore another
mode of binding of inhibitor is the un-competitive mechanism, in this mechanism the
inhibitor irreversibly binds with the enzyme and result an intermediate complex (E-I*)
between the enzyme and the inhibitor. This portion of enzyme is known as allosteric site
(Figure 1). The binding of enzyme with the substrate occurs in an organized fashion and the
substrates have specific affinity toward the enzyme. Though the active site is a specific
region in the enzyme and this specificity depends on the pro-non covalent interaction
available for interaction in the inhibitors. These non covalent interactions between the
enzyme and inhibitor are the main feature for the chemoselectivity of the substrate and
enzymes during formation of the complex.
These interactions mainly provide a surface to regenerate the enzyme for the reaction cycle
as a catalyst. The unique geometrical shapes and topology of the active site is
complimentary for the substrate molecule, which fit like lock and key arrangement. The
enzymes specifically react with only few compounds of the similar structural features, these
structural feature in the inhibitor is known as pharmacohores, while the potency of the
inhibitor is managed by appending various functional groups to the main skeleton of the
inhibitor by making target oriented derivative containing these features and consequently
resulted in the controlled target oriented synthesis of the potent and fascinating inhibitors.
As there are so many theories which explain the mechanistic perspective of the enzymatic
action, only the ‘lock and key’ and the ‘induced fit’ theories are well accepted (Donald et al.,
2005). The mechanistic studies of the biological systems undergoes via a complex
phenomenon, a large number of literature reports are available involving the structurebased design and testing of novel pharmacophores model for the recognition of urease
inhibitors. The intimated details of the molecular geometry of urease enzyme as well as its
mechanism of urea hydrolysis have been confirmed with XRD-analysis.
2.1 Kinetics of enzyme activity
The rate of reaction provide a clue about the affinity of the inhibitor towards the enzyme,
while at the same time it also effect the potency of the inhibitor. The kinetic effect of
rebeprazole has been investigated on the urease inhibition, it is reported that rebeprazole act
as an irreversible noncompetitive inhibitor. Mainly the associated inhibitory potency of
rabeprazole is dependent on the pH of reaction mixture and Ki value which varies at the
progressive inactivation of urease by rabeprazole, initially it proceeds according to pseudofirst-order kinetics with respect to the remaining enzymatic activity at pH 7.0 and 37 °C,
with a second-order rate constant of 0.0017 M-1 s-1. This inhibitor competes with substrate
to bind to the enzyme and form as an E-I (enzyme and inhibitor) complex, that slowly
transformed to an E-I* complex which is stable (Scheme 1) (Park et al., 1996).
The use of non-steady-state kinetic measurements is important to properly analyze the non
competitive inhibition (Mobley et al., 1989; Rosenstein et al, 1984). A number of mechanistic
studies have been carried out on various inhibitors. It is reported that hydroxamic acid which
suggests that a bidentate complex form with one of the nickel ions, quite few scientists
307
Urease Inhibition
H2N
Ni
OH
O
OH
HN
HO
Ni
Ni
O
O
CH3
Ni
CH3
E.I
E.I*
E.I = Enzyme-Inhibitor Complex
E.I*= Stable Enzyme-Inhibitor Complex
Scheme 1. Bi-dentate ligand complex of substrate and enzyme.
believe that it make a bridge between the two nickle ions. Similarly it is also suggested that the
E-I and E-I* type of interactions occurs, forming an initial monodentate species that bridges the
two nickel ions at the active site (Amtul et al., 2002). The non-steady-state studies have also
been employed with other competitive inhibitors, the specific examples for the kinetic analysis
is the interaction of phenylphosphoro diamidate with urease (K. aerogenes) with dissociation
constant of 4.7x10 -5 s-1. The proposed studies suggests that the E-I state where an inhibitor
bound in unidentate mode to one nickel ion while the EI* species involves the bridging of the
two nickel ion with inhibitor where R is the aromatic ring via the formation of tetrahedral
intermediate rather that the hypothetical trigonal bipyramidyl intermediate. Here the
hydrolysis occurs with simple in-line displacement (Andrews et al., 1984).
2.2 Action of inhibition
The modes of action of enzyme have been investigated on the thiol based compounds. As
the presence of an auxochrome sulphar (thio) moiety in the inhibitor help in characterization
via UV-Visible spectroscopy for a variety of enzymes e. g jack bean and K aerogenes ureases,
these spectroscopic perturbation is consistent with the development of thiolate. It is well
documented that the Ni (II) charge-transfer complex is kinetically controlled process. Since a
competition of thiol with urea for the active sites of the enzyme in binding the nickel
provide a relative reference to further study the action of inhibitor over the enzyme.
Ni
OH
Ni
S
HN
H 2N
NH2
A
Ni
B
NH
Ni
Scheme 2. Structural hypothesis for the competitive interaction of the urease active sites
with thiourea (A) and hydroxyurea (B).
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Enzyme Inhibition and Bioapplications
The mechanism of action related to hydrolysis of urease to ammonia and carbamate has
been investigated in depth, the later compound spontaneously decomposes to the carbamic
acid, accompanying the deprotonation generating equilibrium as a result of increase in pH.
The research on urease has contributed extensively towards development of medicinal
organic chemistry (Summer et al., 1926) and crystallization (Amtul et al., 2002). The urea
binds into the pocked adjacent to Ni near Wat-502. The H2-H4 mobile flap may open to
allow access into the active site of enzyme, once the active functional group binds to Ni-1 via
coordination. The urea is then placed in a manner that allows the coordination of its oxygen
with the N epsilon of His 219, by forming hydrogen bond. The urea also accepts a hydrogen
bond from the interaction of its amide nitrogen with side chain of His 320. These non
covalent interactions are oriented to urea in an organized fashion so that the carbonyl group
of the urea undergoes polarization and attacked by hydroxide (Wat-502), that coordinates
other metal atom of Ni-2 and forms tetrahedral intermediate. As one proton is transferred
from His-320 to the nitrogen (leaving group), meanwhile the intermediate collapses into
ammonia and carbamate. The carbamate undergoes hydrolysis to form carbamic acid and
second molecule of ammonia (Amtul et al., 2002). The suboptimal interactions provide a
significant source of substrate binding energy; this energy is commonly found in the
enzymes with mobile active site loops which undergo induced fit. The side chain of His 320
in urease is in close proximity to the side chain of Asp 221 and Arg 336, whereas the
geometry is not optimal for the hydrogen bonding between the residues. His 320 also
interacts with Wat-170 via hydrogen bonding. The Wat-170 also forms hydrogen bond with
two main chain carbonyls. Wat-170 does not have the ability to donate three hydrogen
bonds, so one of the H-bond at any given time is unable to form, making the interactions of
water suboptimal. There are also suboptimal interactions between the nickel metallo center
and solvent. The active site water molecules (Wat-500, Wat, 501, Wat-502) have inter-oxygen
distance that is too short to allow for simultaneous occupancy. Since the occupancy of the
active site is too small to accommodate all three water molecules, there appears a competition
for occupancy at the three positions. All these interactions must contribute to the flexibility of
the mobile flap by the destabilizing the closed positions. These interactions are the main
driving force for the catalytic mechanism of the urease in the various biological processes. The
most of the biochemical reactions occurs by decreasing the free energy difference between the
unbound enzyme with substrate and enzyme-substrate complex which accomplished in two
ways with enzyme binding transition state which is highly favorable or the initial state which
has higher free energy, possibly the addition of suboptimal interactions. Such interaction
makes possible transition state which usually is not favorable, therefore most of the urease
mechanisms involve a number of suboptimal interactions to derive the hydrolysis of urea in
the systems (Mobley et al., 1989; Mulvaney et al., 1981).
3. Potent inhibitors
A vast range of naturally isolated and synthetic urease inhibitors has been reported in the
literature though pharmacological importance and the developed resistance in the bacterial
infections have attracted the attention of the scientific community to explore new versatile
urease inhibitors.
Urease inhibitors are classified broadly into two major classes as metal-organic and organic
compounds, the later examples are such as acetohydroxamic acid, humic acid and 1,4-
309
Urease Inhibition
benzoquinone etc. The inhibitors have broad applications in combination with urea fertilizers
that has been proposed for soil urease activity control (Bundy et al, 1973; Amtul et al., 2002).
There is an example of kinetic studies on quinone-induced inhibition of urease has also been
investigated, one among those is polyhalogenated benzoand naphthoquinones, in which the
quinones were found to be non-competitive inhibitors of jack bean and bacterial ureases of
different strengths (Zaborska et al., 2002; Pearson et al., 1997). Several other potent inhibitors
are being used as first line of treatment for infections causes by urease-producing bacteria
which are available in current market. The most effective inhibitors with safe great potency
profile for considerable control of urease-related ailments are still needed. The available data
provide a basis for urease inhibition properties of thiol-compounds, different cysteine
derivatives (CysDs) with cysteine-like scaffold arylidene barbiturates have been studied as
urease inhibitors such as N-Substituted Hydroxyureas (Amtul et al., 2002; Amtul et al., 2004;
Todd et al., 1989). Acetohydroxamates are the bacterial urease inhibitor and used as
therapeutic potential in hyperammonaemic states (Carlini et al., 2002). The urease inhibitors
have also been employed for the coating of urea in the agricultural soils to reduce the
ammonia mitigating. The nitrogen loss depends on the rate of biodegradation and constant
volatilization in agricultural soils of urea fertilizer, such coating can improve the
bioavailability of nitrogen and consequently increase dry matter yield and nitrogen uptake.
These increases of urea contents in soil result from delayed urea hydrolysis by urease
inhibitors and coating materials. The value of inhibitors in nitrogen mitigating is depending on
the rate of biodegradation and persistence in soils (Amtul et al., 2007). The urease inhibitors
have been investigated for the treatment of bacterial infections and also for the excessive urea
break down in soil, therefore studies on the potent and specific inhibitors have been an active
area of research. Taking in view the great potential of urease in medicine and soil sciences,
several classes of urease inhibitor have been discovered in the recent years (Ara et al., 2007;
Mobley et al., 1989; Mulvaney et al, 1981; Rosenstein et al., 1984). The urease inhibitors can also
be classified into two categories either as metal complexes or organic compounds. The organic
compounds are further classified into the three main classes such as hydroxamic acid analogs,
phosphoramide compounds and thiourea derivatives. Hydroxamic acid (Scheme 3) analogs
are the most commonly known urease inhibitors and first member of its series was
hydroxamic acid which was discovered in 1962. The acetohydraxamic acid with other
numerous derivatives have been synthesized and studied against ureases of plants and
bacterial origin (Mobley et al., 1989). The other members of the series include n-aliphatic
hydroxamic acid, ortho or para substituted benzohydroxamic acid, trans-cinnamoyl
hydroxamic acid with other functional groups and substituents.
O
OH
R
N
H
Scheme 3. General structure of hydroxamic acid.
Phosphoramide compounds are another class of potent inhibitors of the urease enzyme, this
class of inhibitors comprise of a large number of simple and complex compounds. Mainly
310
Enzyme Inhibition and Bioapplications
the less complex examples includes of phosphoramidates and diamidophos phate with
substituted phenylphosphoradiamidates, while a range of N-acyl phosphoric triamidates
(Humayun et al., 2010).
Thio substituted compounds are also strong inhibitors of the urease enzyme. This includes
cysteamine containing a cationic -amino cysteamine, which exhibit highest affinity for the
urease, on the other hand the thiolates containing anionic carboxyl groups are uniformly
poor inhibitors. The pH dependence studies demonstrate that the actual inhibitor is the
thiolate anion (Ansari et al., 2005). A series of bezothiazepines have also been investigated
bearing significant urease inhibitory activities (Kühler et al., 1998). The studies on structure
activity relationship of the thio substituted benzimidazoles with in-vitro and in-vivo efficacy
model, where the potency of the substrate is depend on the substituents positions in the
pharmacophore (Creason et al., 1990). The inhibitory effects of N-(n-butyl) thiophosphoratriamides (NBPT) and its oxon analogs in the soil are also been reported (Zaborska et al.,
2004). There are several other potent inhibitors readily available in the market the most
common is NBPT (N-[n-butyl] thiophosphoratriamide) which is sold worldwide under the
trade name of Agrotain. This fertilizer has amended the coats of urea preventing the urease
enzyme from breaking down the urea for up to fourteen days of life. As this increases the
probability that urea will be absorbed into the soil after a rain rather than volatilized into the
atmosphere which reduce the nitrogen content in the soil. The rich content of urea in soils
causes hydroxylation under the surface of soil and decreases atmospheric losses of the major
ingredients of fertilizers. The use of potent inhibitors also decreases the localized zones of
high pH which is common in untreated urea agricultural fields. These remedies are
commonly employed to maintain the pH of the soil and controlled ammonia volatilization.
The use of inhibitors increases the efficiency of urea in delivering nitrogen to soil.
A wide range of metal complexes with meager to good urease inhibitory activities are reported
which includes heavy metal ions, such as Cu2+, Zn2+, Pd2+ and Cd2+ (Zaborska et al., 2001; Asato
et al., 1997). There are reports on the urease inhibitory activities of organotin(IV), vanadium(IV),
bismuth(III), copper(II), and cadmium(II) complexes (Khan et al., 2007; Ara et al., 2007; Hou et
al., 2008; You et al., 2008; Tanaka et al., 2003; Cheng et al., 2007; You et al., 2010). A series of
metal complexes of Schiff bases such as thiocyanato-coordinated manganese(III) complexes
based on N-ethylethane-1,2-diamine and N-(2-hydroxyethyl)ethane-1,2-diamine have
shown significant urease (Huang et al., 2011). There is library of compounds are available in
various databases and the recent developments is continuously adding numerous class of of
hetero-dinuclear CuII–ZnII complexes (You et al., 2011).
4. Structure Activity Relations (SAR)
The hydroxamic is the most commonly used urease inhibitor coordinating to nickel of
urease through carboxylic acid and hydroxyl amine (Scheme 3). The structural studies on
the urease from Klebsiella aerogenes, Bacillus pasteurii, and Helicobacter pylori have revealed
that it contains a dinuclear Ni active site with a modified amino acid side chain-containing a
carbamylated lysine residue that bridges the deeply buried metal atoms as shown in Scheme
3 (Benini et al., 1999; Jabri et al., 1995; Ha et al., 2001). The crystal structure has been resolved
only for bacterial ureases; it has an active centre which contains two simple coordinated
water molecules and a bridging OH group. The specificity of the enzyme is closely related to
the shape of its active centre (Mobley et al., 1995).
311
Urease Inhibition
NLys
O
NHis
Ni
NHis
H2O
O
O
H
Ni
H2O
NHis
NHis
O
Asp
O
Scheme 4. Active center of urease.
There is vast reports on the structure of urease, the Klebsiella areogenes, the urease structure has
a trimer of alpha, beta and gamma units in a triangular arrangement with three active sites per
enzyme. These subunits include a 60.3 KD apha, a 11.7 KD beta and a 11.1 KD gamma subunit
(Jabr et al., 1992). The interactions between the individual subunits of all three α subunits make
extensive contacts with each other to build a trimer. Each alpha subunit packs between two
beta and two gamma subunits to form the side of the triangle arrangement (Jabri et al., 1995).
Each beta subunit packs between the two of the adjacent alpha subunits at each corners of the
triangle. The remaining gamma subunit interacts with two gamma subunits at the
crystallographic three fold axis. Because of the high degree of interaction, it is not obvious that
which alpha, beta and gamma chains make up the primary units. High conservation of
sequences and the extensive interactions of the trimer suggest a similar trimer structure in all
the urease which is further subdivided into sub-units. The gamma subunit consists of an
alpha-beta domain with four helices and two anti parallel stands. Two helices (b and c) and the
two strands pack together with right-handed up-down-up-down topological order. One of the
remaining helices (a) is lined up peripherally and interacts at the three fold axis with the other
two gamma subunits, tightly packing against itself and to other (a) helices in an orthogonal
manner. The last helix (d) experience single turn. The first fifteen residues of the beta subunits
form two antiparallels beta sheets with the amino-terminal residues of the alpha subunits. The
core of the beta subunit forms an imperfect six-stranded antiparallel beta sheet (Mobley et al.,
1989; Hausinger et al., 1993). This subunit stabilizes the trimer by interacting with domain two
of its own alpha subunits and domain one of a symmetry related alpha subunit. The alpha
subunit consists of two domains, an (alpha, beta) 8 barrel domain and a primarily beta
domain. The (alpha, beta) 8 barrel is the only domain that contributes to the active site. Its
barrel is rather elliptical with the long axis connecting strands 1 and 7. Strands 9 and 10 extend
the lower portion of strands 1 and 3. The active site is located at the carboxyl terminal of the
strands. A flap is formed that covers the active site by a helical excursion between strand 7 and
helix 7. Mixed four-strands and eight-stranded beta sheets form the wall of U-Shaped canyon
which makes up the second beta domain. The walls are connected by the long strands 5 and 6
together with strands 10 and 11 which go down on one side, and up on the other side. The
structure activity relationship of the various urease inhibitors are reported in literature, the
studies on in-vitro with in-vivo efficacy of the effect benzimidazoles suggested that the
orientation and position of the pyridyl moiety larger and more lipophillic chromophore of the
312
Enzyme Inhibition and Bioapplications
substrate has affinity which affect the minimum bacterial concentration of the helicobacter
pylori (Kühler et al., 1998).
5. Conclusion
The chapter is focused on the importance and future implications of the urease. The
emphases have been laid on urease keeping in mind its role as virulence factor for the
urinary tract infections and gastrointestinal infections while the inhibition of urease is also
effective in the agricultural sector. Urease enzyme inhibition has become a growing area of
research at the interface of the biomedical sciences. The broad range of potent inhibitors has
been reported in literature from simple organic molecules to complex molecular building
block with metal complexes. These molecular building blocks are expected to bring
revolution in pharmaceutical, agricultural and environmental fields. Urease based sensors,
adsorbent and other devices have also been discussed that are commercially used.
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